LIBRARY 


UNIVERSITY  OF  CALIFORNIA. 


Class 


Qualitative  Analysis 


Martin 


An  Introduction 

to  the 


Theory  and  Practice  of  Qualitative 
Analysis  by  Solution 


BY 
F.  W.  MARTIN,  Ph.  D., 

Professor  of  Chemistry  in  the  Randolph-Macon  Woman's  College 


Lynchburg,  Virginia 

J.  P.  Bell  Company,  Inc.,  Publishers 

1907 


[^ 


7  C 


f 


ENERAL 


This  book  is  dedicated  with  the  highest  esteem 

to 

Edward  F.   Bartholomew  and  Wilhelm   Oswald 
my  first  and  last  masters  in  science. 


Copyright,  1907,  by  F.  W.  Martin, 
Lynchburg,  Va. 


PREFACE 


The  study  of  Qualitative  Analysis  has  latterly  fallen  somewhat  into  disrepute 
in  American  colleges.  I  believe  that  the  reason  for  this  is  three-fold :  1st.  The 
empirical  treatment  given  the  subject  until  recently  in  lectures  and  texts,  result- 
ing in  the  latter  instance  in  such  remarkable  complication  that  I  have  never  yet 
seen  an  undergraduate  who  could  read  one  without  assistance.  2nd.  The  queer 
compositions,  called  Unknowns,  that  are  generally  given  students  for  practice. 
Casuistically  speaking,  these  mixtures  might  be  worshipped — as  it  may  be  safely 
assumed  that  some  do — inasmuch  as  they  are  not  the  likeness  of  anything  that  is. 
3rd.  The  fact  that  the  teaching  of  this  branch  of  Chemistry  is  frequently  com- 
mitted to  the  hands  of  young  instructors  whose  knowledge  and  pedagogic  skill 
are  not  always  adequate  to  the  demands,  often  great,  made  upon  them. 

A  score  of  years'  experience  convinces  me  that  Qualitative  Analysis  may  be 
made  to  yield  discipline  of  a  high  order;  its  utility  as  a  tool  in  the  prosecution 
of  chemical  investigations  is  admitted  by  nearly  every  one.  I  have  therefore 
prepared  this  little  book  in  the  hope  that  I  might,  through  it,  be  able  to  some- 
what promote  study  of  this  subject.  I  do  not  claim  that  this  treatise  is  so  plain 
that  a  wayfarer,  though  a  Freshman,  need  not  err  therein,  nor  that  it  will  either 
dispense  with  the  instructor  or  make  his  post  a  sinecure.  However,  I  have  sin- 
cerely endeavored  to  render  the  plan  simple,  the  philosophy  scientific,  and  the 
scheme  of  analysis  so  obvious  that  students  of  average  abilities  will  be  able  to 
acquire  from  the  book  a  creditable  knowledge  of  the  subject;  as  for  the  others — 
gegen  die  Dummheit  sclbst  die  Goiter  kampfen  vergebens.  There  will  always  be  a 
field  for  the  instructor.  Indeed,  he  is  the  one  indispensable  feature  of  a  labora- 
tory. While  the  methods  employed  are  drawn  from  the  classic  of  Fresenius,  the 
explanations  are  derived  from  the  modern  theory  of  solutions;  and  I  have 
sought  to  render  every  step  of  the  way  continuously  rational  to  the  learner  by 
means  of  cross  references. 

I  wish  to  record  here  my  conviction  that  Qualitative  Analysis,  in  whatever 
state  of  dilution,  is  not  adapted  to  the  curriculum  of  secondary  schools.  The 
study  is  too  abstruse  for  immature  students.  As  a  rule,  they  can  at  best  only 
learn  to  make  a  set  of  test-tube  manipulations  by  rule-of-thumb.  If  they  proceed 
to  college,  this  experience  will  often  be  found  encysted  as  a  wen  of  conceit  which 
must  be  cut  out  before  healthy,  intellectual  growth  can  take  place.  Possibly 
herein  may  be  found  a  fourth  reason  why  this  branch  of  Chemistry  has  suffered 
declension  in  the  esteem  of  college  teachers. 

COLLEGE  PARK,  JULY,  1907. 


17498:? 


CONTENTS 


CHAPTER  I. 

INTRODUCTION. —  Chemistry  defined.  Qualitative  Analysis  defined.  The  iden- 
tification of  substances.  Precautions  to  be  observed. 

CHAPTER  II. 

THEORY  OF  SOLUTIONS. —  Solution  defined.  Kinds  of  solution.  Saturation. 
Solubility.  Osmosis.  Vapor  pressure.  lonization.  Classes  of  electro- 
lytes. Degree  of  dissociation.  Modes  of  ionization.  Physical  solution. 

CHAPTER  III. 

THEORY  OF  SOLUTIONS  :  METATHESIS. —  Solvents.  Chemical  activity.  Chem- 
ical reactions.  Metathetical  ionization.  Chemical  equilibrium.  Kate  of 
reaction.  Concentration  of  a  solution.  Change  of  equilibrium.  Hydrol- 
ysis. Completion  of  reactions. 

CHAPTEK  IV. 
ANALYTIC  GROUPS. —  The  course  outlined.     The  groups. 

CHAPTEK  V. 
IDENTIFICATION  OF  THE  BASIC  IONS. 

CHAPTER  VI. 

SYSTEMATIC  ANALYSIS  FOR  THE  ACIDIC  TONS. 

CHAPTER  VII. 
SYSTEMATIC  ANALYSIS  FOR  THE  BASIC  IONS. 

CHAPTER  VIII. 
PRACTICAL  EXERCISES. 


APPENDIX 


REAGENTS. 


Qualitative  Analytical  Chemistry 


CHAPTER   I 


INTRODUCTION 

1.  Chemistry    defined. — Chemistry    is  that    branch  of   science 
which  treats  of  those  properties  of  bodies    which    are    functions   of 
their  composition  and    constitution.     That    division    of  the   subject 
which  teaches  how  to  ascertain  the  composition  of  bodies  is  called 
Analytical  Chemistry.     By    analysis    we    may    find    what    elements 
are  present  in  a  compound,    and   also  the  relative  amount  of  each. 
The    first    of    these    processes    is    called    qualitative  ;    the  second, 
quantitative. 

2.  Definition  of  Qualitative  Analysis — Qualitative  Analysis 
is,    consequently,    that    branch    of  Analytical    Chemistry   which  is 
devoted  to  methods  for  finding  what  chemical  elements  (and,   some- 
times,   radicals,    or    compounds)    are    present    in    a    body.     This 
definition  is  comprehensive.     The  present  treatise  will  be  restricted 
to    a    study    of  the    more    abundant    elements  and  their  common 
compounds,    chiefly    inorganic,   as   they  are  likely  to  be    found  in 
nature  and  in  the  products  of  manufacture. 

3.  The   identification   of   substances. — Chemical  analysis    is 
both  a  science  and  an  art.     We  identify  material  things  by  means 
of  characteristic  trains  of  sense  impressions.     The  simplest  case  of 
recognition    is    amazingly   complex.     The    human    infant    requires 
months    of   association    under    constant    instruction    to  enable  it   to 
detach    its    own    species   from   the    universe   and  isolate  itself  as  a 
conscious    unit.     If  the    child    be   given    at  this   stage   a   lump  of 
loaf  sugar   and    told    at    the   same   moment    the     word     sugar,    it 


0  QUALITATIVE    ANALYSIS 

receives  through  the  eye  an  impression  of  color  ;  through  the 
hand,  the  ideas  of  hardness  and  weight;  through  the  eye  and  the 
hand  conjointly,  the  percepts  of  form  and  dimension;  and,  finally, 
the  organs  of  taste  and  digestion  add  those  sensations  of  sweetness  and 
satisfaction  which  complete  the  concept  sugar.  A  repetition  of  the 
process  at  intervals  during  a  few  weeks  establishes  a  train  of  nerve- 
cell  adjustments  whose  recognition  by  the  percipient  is  called  memory; 
and  thereafter  this  entire  series  of  cell  adjustments  may  be  produced 
by  a  renewal  of  but  one  or  two  of  the  original  sensations,  the  remainder 
being  automatically  interpolated.  After  a  short  time  the  child  is 
able  to  identify  sugar  quickly  and  positively.  From  this  experience 
as  a  point  of  departure,  it  proceeds  in  like  manner  to  differentiate  the 
generic,  sugar,  into  the  specific  varieties — loaf  sugar,  granulated 
sugar,  molasses,  etc.  And  its  acquaintance  with  the  world  at  large  is 
gradually  extended  by  the  same  empirical  process. 

If  later  the  child,  now  a  youth,  undertakes  the  study  of  Chemistry, 
he  will  familiarize  himself  with  elements  and  compounds  by  precisely 
the  same  method.  Expertness  in  differentiating  and  classifying  such 
substances  is  dependent,  primarily,  upon  the  delicacy  of  adjustment 
of  the  nervous  system,  that  is,  upon  talent;  and,  secondarily,  upon 
practice.  In  so  far,  chemical  analysis  is  an  art.  But  the  task  of  the 
chemist,  which  is  no  less  than  to  acquire  a  positive  acquaintance  with 
the  ultimate  composition  and  characteristic  qualities  of  every  accessi- 
ble body  in  the  material  universe,  is  greatly  abridged  and  lightened 
by  the  classification  of  phenomena,  substances,  and  bodies  by  certain 
principles  which  render  the  world  rational  to  us,  and  which  we  name 
natural  laivs.  In  this  aspect,  chemical  analysis  is  a  science. 

4.  Precautions  to  be  observed — For  the  reasons  adduced,  the 
student  should  not  expect  immediate  and  invariable  success  in  his 
laboratory  determinations.  Expertness  comes  only  from  long  and 
intelligent  practice.  Moreover,  no  process  of  analysis  is  perfect,  be- 
cause nothing  of  human  invention  is  perfect.  Consequently,  it  is 
necessary  for  the  student  to  exercise  scrupulous  cleanliness  and  rigor- 
ous care  in  carrying  out  directions,  and  to  have  thorough  knowledge 
of  what  he  is  about. 


CHAPTER  II 

THEORY  OF  SOLUTIONS 

5.  The  identification  of  a  substance  necessitates  its  being  carried 
through  a  series  of  transformations  (§3)  whereby  the  individual  com* 
ponents  are  made,  at  the  will  of  the  experimenter,  to  assume  new: 
combinations,  in  which,  one  of  the  components  being  known,  the  other 
is  recognized  by  the  specific  properties  of  the  whole  newly -formed 
compound.     Such  transformations  are  generally  effected  most  readily 
in  aqueous  solutions. 

6.  Solution  defined. — The  substantive,  solution,  signifies  a  mixture 
whose  components  are  in  equilibrium  and  incapable  of  being  com- 
pletely separated  by  ordinary  mechanical  processes.     The  substance 
in  which  solution  takes  place  is  called  the  solvent;  the  substance  which 
is  dissolved  is  called  the  solute.     Solution  may  occur  either  with  or 
without  metathesis.     In  the  present  chapter,  only  the  latter  kind  of 
solution  will  be  discussed. 

A  solute  is  said  to  be  dissolved  in  a  solvent  when  it  becomes  molec- 
ularly  diffused  throughout  it.  The  solute  may  be  uniformly  distrib- 
uted throughout  the  solvent  so  as  to  form  a  homogeneous  mixture; 
but,  as  a  rule,  this  is  not  the  case  when  an  interval  of  time  has 
elapsed  after  thorough  agitation. 

No  solvent  is  known  that  is  not  to  some  extent  soluble  in  its  solute; 
therefore,  in  speaking  of  solutions  it  is  necessary  to  specify  which  is 
taken  as  solvent  and  which  as  solute.  This  may  be  done  by  placing 
the  name  of  the  solvent  after  the  preposition  in,  thus — "Dissolve 
alcohol  in  water." 

7.  Kinds  of  solution. — Different  kinds  of  solution  may  occur. 
We  may  have  a  solution — (1)   of  a  gas  in  a  gas;  (2)  of  a  gas  in  a 
liquid;    (3)   of  a  gas  in  a  solid;   (4)   of  a  liquid  in  a  gas;  (5)  of  a 
liquid  in  a  liquid;  (6)  of  a  liquid  in  a  solid;  (7)  of  a  solid  in  a  gas; 
(8)  of  a  solid  in  a  liquid;  (9)  of  a  solid  in  a  solid. 


QUALITATIVE    ANALYSIS 

(1)  The  solution  of  one  gas  in  another  is  illustrated  by  the  dif- 
fusion of  atmospheric  oxygen  in  nitrogen.     In  practice,  this  kind  of 
solution  is  usually  effected  by  introducing  the  solute  into  the  solvent 
retained  in  a  flask  or  bottle  by  means  of  a  water  or  a  mercury  bath. 
The  solute  penetrates  the  solvent  till  uniform  admixture  results.  The 
properties  of  the  mixture  are  the  sum  of  the  properties  of  the  con- 
stituents.    Hence,    the  expression  of  the  simple-gas  law,  PV=RT, 
transformed  for  a  gas-solution  becomes  PV=pJv1-fp,v2-j-p8v81}-  ...=RT. 
This  is  Dalton's  law. 

(2)  The  solution  of  a  gas  in  a  liquid  is  illustrated  by  hydrogen 
sulphide  in  water.     This  kind  of  solution  is  best  accomplished  by 
passing  the  gas  into  the  fluid  by  means  of  a  delivery  tube  and  causing 
minute  bubbles  to  ascend  through  the  greatest  depth  of  solvent  ob- 
tainable under  the  circumstances.     Obviously  the  operation  will  be 
accelerated  by  keeping  the  solvent  in  motion.     The  amount  of  gas 
that  will  be  absorbed  depends  primarily  upon  the  nature  of  the  gas 
and  of  the  liquid,  different  liquids  absorbing  different  amounts  of  the 
same  gas,  and  secondarily  upon  pressure  and  temperature,  being  a 
direct  function  of  the  pressure  and  an  inverse  function  of  the  temper- 
ature.    This  is  Henry's  law.     Mixed  gases  are  absorbed  directly  in 
proportion  to  their  partial  pressures  in  accordance  with  Dalton's  law. 

(3)  The  solution  of  a  gas  in   a  solid  is  illustrated  by   hydrogen 
in  palladium.     Since  it  has  been  shown  that  gases  dissolve  in  any 
solvent  in  the  direct  ratio  of  their  pressure,  this  variety  of  solution 
also  obeys  the  laws  of  Henry  and  Dalton. 

(4)  The  solution  of  a  liquid  in  a  gas  is  illustrated  by  the  absorption 
of  water  in  air.     This  kind  of  solution  is  accomplished  by  allowing 
the^liquid  to  stand  some  time  in  a  suitable  vessel  containing  the  gas  or 
by  passing  a  stream  of  gas  through  the  liquid.     Dalton  discovered 
that  a  liquid  in  the  presence  of  a  gas  exerts  the  same  vapor-pressure 
as  when  in  a  vacuum;  hence  this  kind  of  solution  proceeds  to  satura- 
tion in  direct  ratio  to  the  temperature. 

(5)  The  solution  of  a  liquid  in  a  liquid  is  illustrated  by  alcohol  in 
water.     This  kind  of  solution  is  practically  accomplished  by  pouring 


QUALITATIVE    ANALYSIS  9 

the  solute  into  the  solvent  and  stirring  or  shaking  the  mixture.  Two 
cases  are  involved:  (a)  solute  and  solvent  mix  in  all  proportions;  (b) 
the  liquids  are  only  partially  miscible.  Physically,  the  properties  of 
a  mixture  belonging  to  class  (a),  unlike  the  corresponding  class  of 
gas-solutions,  are  not  the  sum  of  the  properties  of  the  constituents. 
Thus,  the  volume  of  the  mixture,  V,  is  never  equal  to  the  sum  of  the 
volumes  of  the  constituents,  v^  v2...,  but  is  either  less  or  greater. 
Class  (b)  is  physically  still  more  complex;  but  since  at  some  elevated 
point  of  temperature  solutions  of  class  (b)  pass  over  into  class  (a), 
the  former  may  be  regarded  as  a  variety  of  the  latter. 

(6)  The  solution  of  a  liquid  in  a  solid  is  illustrated  by  mercury  in 
zinc.     It  appears  from  the  studies  of  van't  Hoff  that  such  solution  is 
governed  by  the  same  laws  as  that  of  liquids  in  liquids. 

(7)  The    non-metathetical    solution   of  solids    in  gases    is   called 
sublimation.     The  vaporization  of  iodine  in  air  illustrates  this  variety 
of  solution.     The  sublimation  pressure  varies  directly  with  the  tem- 
perature up  to  the  melting  point  of   the  solid.     The  properties  of 
such  a  solution  are  the  sum  of  those  of  its  constituents. 

(8)  The  solution  of  a  solid  in  a  liquid  is  illustrated  by  sodium 
chloride  or  cane  sugar  in  water.     In  practice,  such  solution  is  facili- 
tated by  pulverizing  the  solid  and  adding  it  to  the  solvent  with  stir- 
ring or  shaking.     The  operation  is  helped  by  heating  the  solvent  if 
endothermy  takes  place  and  by  cooling  it  if  exothermy  occurs.     In 
this  kind  of  solution,  the  influence  of  pressure  is  that  of  an  inverse 
function;  but  since  all  qualitative  operations  are  carried  on  at  atmos- 
pheric pressure,  this  agency  may  be  neglected. 

(9)  The  solution  of  a  solid  in  a  solid  is  illustrated  by  the  penetra- 
tion of  carbon  into  iron  as  formerly  practiced  in  making  steel  by 
heating  charcoal  and  iron  in  earthenware  boxes,  and  by  the  fact  that 
if  a  piece  of  zinc  be  electroplated  on  one  side  with  copper  the  zinc 
gradually   penetrates   the  copper  and   whitens  it.     This  variety  of 
solution  appears   to  follow  the  same  laws  as  does  the  solution  of  a 
liquid  in  a  liquid. 


10  QUALITATIVE    ANALYSIS 

8.  Saturation. — When  a  solvent  contains  all  the  solute  that  it  can 
dissolve  at  a  given  temperature  and  pressure  it  is  said  to  be  saturated. 
Variation  in  either  temperature  or  pressure  will  produce  in  such  a 
solution  either  a  deposition  or  a  further  absorption  of  the  solute,  as 
explained  above.     However,  the    condition    of  saturation    does   not 
invariably  follow  instantly  changes  in  the  conditions  under  discussion. 
Thus,  a  saturated  solution  whose  temperature  is  lowered  or  pressure 
reduced  may  only  deposit  its  excess  of  solute   after   an    appreciable 
interval.     Nevertheless,  the  equilibrium  of  the  mixture  is  rendered  at 
once  unstable,  and  it  will  finally  pass  to  a  state  of  stable  equilibrium 
corresponding  to  the  new  conditions.     When  such  change  of  con- 
ditions would  theoretically  necessitate  a  separation  of  solute  that  does 
not  occur,  the  solution  is  said  to  be  supersaturated. 

9.  Solubility. — The  solubility  of  a  substance  is   the   number  of 
grams  thereof  which  100  grams  of  the  solvent  can  dissolve  at  a  given 
temperature  under  the  pressure  of  one  atmosphere. 

10.  Osmosis. — If  an  aqueous  solution  be  covered  by  a  layer  of  pure 
water,  the  solute  begins  at  once  to  diffuse  into  it;  and  this  operation 
continues   until   the  mixture  becomes  homogeneous  throughout  the 
entire  mass.    However,  the  migration  of  the  solute  may  be  arrested  by 
interposing  between  the  two  portions  of  the  solvent  a  wall  that  is 
permeable  by  the  solvent  but  not  by  the  solute.     Such  partitions  are 
called  semipermeable.     They  may  be  made  of  certain  animal  or  veg- 
etable tissues,  or  layers  of  amorphous  chemical  compounds,  as  ferro- 
cyanide  of  copper,  laid  down  by  chemical  reaction  in   earthenware 
cells.   When  such  a  cell,  fitted  with  a  manometer,  is  filled  with  a  so- 
lution and  placed  in  pure  water,  the  latter  begins  at  once  to  pass  into  the 
cell.   Since  none  of  the  solute  can  escape  from  the  cell,  the  pressure 
therein  is  increased  and  registered  by  the  manometer.     If  the  tem- 
perature be  kept  constant,  the  operation  goes  forward  to  a    maxi- 
mum which  is  directly  proportional  to  the  strength  of  the  solution. 
The  ingress  of  water  can  be  hindered  by  exerting  a  pressure  upon  the 
solution  in  the  cell  in  the  opposite  direction.    When  such  pressure  be- 
comes equal   to  that  exerted  by  the  water  passing  into  the  cell,  mi- 


QUALITATIVE    ANALYSIS  11 

gration  of  water  ceases.  The  force  exerted  to  accomplish  this,  it  will 
readily  be  seen,  is  capable  of  being  exactly  measured ;  it  is  an  equiva- 
lent of  the  counter  pressure  of  the  solution,  and  its  value  is  called  the 
osmotic  pressure  of  the  solution.  In  practice,  the  value  of  the  osmotic 
pressure  is  instantly  determined  by  taking  the  maximum  reading  of 
the  manometer. 

It  thus  appears  that  a  substance  in  solution  behaves  as  though  it 
were  in  the  gaseous  state.  For,  by  Boyle's  law,  the  pressure  exerted 
by  a  gas  is  proportional  to  the  volume,  i.  e.,  the  concentration. 
Hence  the  gas  equation,  pv=RT,  applies  to  all  solutions.  The  osmotic 
pressure  is,  therefore,  proportional  to  the  mass  of  the  solute  and 
always  the  same  for  a  given  solvent;  but  its  value  varies  with  different 
solvents.  Hence,  the  gas  equation  generalized  for  solutions  becomes 
pv=i  RT,  i  being  a  constant  factor  for  the  solvent  employed  and  p 
being  the  osmotic  pressure. 

ii.  Vapor  pressure. — It  is  a  familiar  fact  that  a  liquid  exposed  to 
the  air  will  continuously  give  off  vapor  till  the  whole  has  evaporated. 
If,  however,  the  liquid  be  in  a  stoppered  bottle,  evaporation  will  go 
steadily  forward,  at  constant  temperature,  till  the  contained  air  is 
saturated.  At  this  point,  the  pressure  of  the  vapor  upon  the  internal 
walls  of  the  bottle,  including  of  course  the  free  surface  of  the  liquid, 
is  equal  to  the  force  operating  between  the  particles  of  the  liquid 
tending  to  drive  them  through  the  surface  of  the  fluid.  The  former 
force  is  called  vapor  pressure;  the  latter,  vapor  tension. 

The  vapor  pressure  of  a  liquid  may  be  measured  at  different  tem- 
peratures with  a  manometer.  Since  at  the  boiling  point  vapor  pressure 
and  tension  are  equal,  we  may  measure  variations  in  tension  by  no- 
ting variations  in  boiling  point  at  the  constant  pressure  of  the  atmos- 
phere. 

Now  the  vapor  tension  of  a  solvent  is  lowered  by  the  introduction 
of  a  solute,  so  that  the  boiling  point  of  a  solution  is  always  higher  than 
that  of  the  pure  solvent;  or,  conversely,  the  freezing  point  of  a  solution 
is  lower  than  that  of  the  pure  solvent.  The  lowering  of  the  vapor 
tension  has  been  found  to  be  in  proportion  to  the  mass  of  solute  present. 


12  QUALITATIVE    ANALYSIS 

Hence,  it  appears  that  evaporation  is  a  variety  of  osmosis  in  which  the 
free  surface  of  the  liquid  plays  the  role  of  a  sernipermeable  membrane; 
it  is  permeable  for  the  solvent  but  not  for  the  solute. 

If  we  now  substitute  equimolecular  masses  for  equal  masses  where 
the  latter  words  occur  in  the  foregoing  discussion,  it  appears  that  the 
lowering  of  vapor  tension  {decrease  of  osmotic  pressure^)  in  a  given  sol- 
vent produced  by  the  solution  of  gram-molecular  weights  *  of  different 
solutes  is  a  constant.  This  fact  is  utilized  in  the  determination  of 
molecular  weights  by  the  well  known  equation  M=c— ^-,  where  M  is 
the  molecular  weight  of  the  dissolved  substance,  s  its  weight  in 
grams,  A  is  the  observed  elevation  of  the  boiling  point  (or  depres- 
sion of  the  freezing  point),  L  is  the  weight  in  grams  of  the  solvent 
used,  and  c  is  the  constant  for  molecular  depression  depending  upon 
the  particular  solvent  employed.  But  the  discussion  is  introduced 
here  in  order  to  lay  a  foundation  for  the  matter  presented  in  the  next 
section. 

12,  lonization. — Several  classes  of  compound  substances  when  dis- 
solved in  water  (and  in  a  few  instances  in  certain  other  solvents)  do 
not  conform  to  the  rule  enunciated  in  the  last  paragraph.  Their 
molecular  weights,  determined  from  aqueous  solutions  by  the  boiling 
point  and  freezing  point  methods,  are  approximately  small  multiples 
(2-5)  of  what  they  should  be.  From  the  fact  that  hydrochloric  acid 
gas  is  made  from  its  constituent  gaseous  elements  reacting  by  volume, 
thus — 

HH-fClCl=HCl+HCl, 

and  the  weights  of  the  reacting  volumes  being  respectively  2  and  71, 
we  conclude  that  the  molecular  weight  of  HC1  is  36.5.  But  the 
depression  of  vapor  tension  in  an  aqueous  solution  of  HC1  yields  a 
value  for  the  constant  c,  (almost)  twice  as  great  as  it  should  be  to  corre- 
spond with  the  accepted  molecular  weight.  Assuming  that  the  solu- 
tion law  (§11)  holds  good  in  this  case,  the  only  possible  explanation 
of  the  phenomenon  is  the  dissociation  of  the  molecules  of  the  solute, 

*A  gram-molecule,  or  mol,  of  a  substance  is  its  molecular  weight  in  grams;  e.  g.,  a 
gram-molecule  of  NaCl  is  58.5  grams.  In  practice,  we  may  employ  a  convenient 
fractional  part  of  this. 


QUALITATIVE    ANALYSIS  13 

one  molecule  of  hydrochloric  acid  yielding  two  new  sub-molecules,  one 
of  hydrogen  and  one  of  chlorine.  The  correctness  of  this  assumption 
is  shown  by  the  fact  that  all  solutions  which  manifest  an  abnormal 
lowering  of  vapor  tension  are  conductors  of  the  electric  current  while 
other  solutions  and  pure  water  are  not. 

Substances  whose  aqueous  solutions  conduct  the  electric  current  are 
comprised  in  the  three  familiar  classes  of  chemical  compounds — acids, 
bases,  salts.  They  are  called,  comprehensively,  electrolytes.  If  the 
electrodes  of  a  voltaic  cell,  or  other  source  of  the  electric  stream,  be 
introduced  into  the  solution  of  an  electrolyte,  its  hydrogen  or  metal  is 
separated  and  deposited  on  the  cathode,  and  simultaneously  the  non- 
metal  or  non-metallic  radical  appears  at  the  anode.  The  explana- 
tion of  this  behavior  can  be  found  only  in  the  fact  that  opposite  kinds 
of  electricity  attract.  Now,  the  anode  (the  electrode  from  which  the 
current  travels)  is  charged  with  positive  electricity,  and  the  cathode 
(the  electrode  to  which  the  current  flows)  is  charged  with  negative 
electricity.  Hence,  we  conclude  that  dissociated  hydrogen  and  metal 
atoms  and  metallic  radicals  are  charged  with  positive  electricity,  and 
that  not-metal  atoms  and  non-metallic  radicals  are  charged  with  neg- 
ative electricity.  The  particles  that  migrate  under  influence  of  the 
electric  current  are  called  ions.  Ions  that  carry  charges  of  positive 
electricity  are  called  positive  ions  or  cations;  those  that  carry  charges 
of  negative  electricity  are  called  negative  ions  or  anions.  Since  pure 
water  does  not  conduct  the  current,  it  is  obvious  that  ionization  in  so- 
lutions is  caused  by  the  water  only.  This  is  called  electrolytic  disso- 
ciation or  ionization.  The  separation  of  ions  from  solutions  by  means 
of  the  electric  stream  is  called  electrolysis.  Since  electrolysis  establishes 
the  fact  that  molecules  of  electrolytes  are  dissociated  by  solution  into 
two  or  more  independent  parts,  we  have  in  the  existence  of  ions  the 
explanation  of  the  abnormal  vapor  tension  exhibited  by  such  com- 
pounds. 

13.  Classes  of  electrolytes  defined. — An  ion  differs  from  an  ordi- 
nary atom  or  radical  in  that  it  is  a  free  particle  and  has  associated  with 
it  a  charge  of  electricity.  Hence,  the  symbol  for  an  ion  is  written 


14 


QUALITATIVE    ANALYSIS 


with  electricity  symbols  added,  thus — H+,  Na+,  Ca++;  Cl  ,  O  , 
NO  3.  The  valence  of  an  ion  is  indicated  in  its  symbol  by  the 
number  of  electricity  charges.  Since  in  aqueous  solution,  acids  dis- 
sociate into  H+  and  a  not-metal  radical,  and  bases  dissociate  into 
OHT  and  a  metal  radical,  and  salts  dissociate  into  a  metal  radical, 
M+,  and  a  not-metal  radical,  non-M~,  we  may  define  these  classes  of 
electrolytes  as  follows : 

An  acid  is  a  compound  whose  water  solution  contains  free  hydrogen 
ions. 

A  base  is  a  compound  whose  water  solution  contains  free  hydroxyl 
ions. 

A  salt  is  a  compound  whose  water-solution  contains  free  metal  or 
metallic-radical  ions  and  free  not-metal  or  not-metallic-radical  ions. 
Or,  more  briefly,  a  salt  is  a  compound  whose  water  solution  contains 
both  basigen  and  acidigen  ions. 

These  definitions  are  illustrated  in  the  subjoined  diagram. 


In  general, 


In  general  (basigen  radical) 


H+ 

cr 

H+ 

NO 

H+ 

C2P 

H+ 

R" 

Na+ 

OH 

M+ 

OH 

2HJ 

OH 

R  + 

OH 

Na+ 

cr 

M+ 

R" 

R+ 

R" 

(acidigeu  radical) 


(acidigen  radical) 


14.  Degree  of  dissociation. — Inasmuch  as  the  ionization  of  an 
electrolyte  is  caused  by  the  process  of  solution  (§  12),  it  follows  that 
the  degree  of  ionization  varies  directly  as  the  dilution ;  and  at  infinite 


QUALITATIVE    ANALYSIS  15 

dilution,  it  becomes  infinitely  great.  The  degree  in  a  given  case  may 
be  ascertained  by  measuring  the  electric  conductivity  of  the  solution 
according  to  methods  given  in  treatises  on  Physics.  It  has  thus  been 
ascertained  that  equivalent  solutions*  of  most  of  the  ordinary  electro- 
lytes are  practically  completely  dissociated  upon  reaching  a  thousand- 
fold dilution.  Exactly  defined,  the  degree  of  dissociation  (x)  of  a  dis- 
solved electrolyte  at  any  stage  of  dilution  is  equal  to  the  ratio  of 
molecular  conductivity  at  this  stage  (ju)  to  the  molecular  conduc- 
tivity at  infinite  dilution  (/^  ),  or 


15.  Modes  of  ionization.  —  Molecules  of  electrolytes  break  down 
into    an    equivalency    of  cations  and  anions  —  thus  HC1=H+-|-C1~, 
KOH=K++OH~  Ca(OH)2=Ca++-f  OH~+OH~,  etc.  Other  modes 
will  be  discussed  later  (§  20). 

16.  Physical  solution.  —  The  kinds  of  solution  discussed  in  this 
chapter  are  commonly  termed  physical.     Physical  solution,  it  will  be 
remarked,  embraces  the  solution  of  electrolytes  and  non-electrolytes 
in  all  cases  not  accompanied  by  metathesis. 

*Equivalent  solutions  are  those  containing  in  equal  volumes  those  weights  of 
reagents  which  react  with  one  another  upon  a  hydrogen  basis,  e.  g.,  40g.Na  OH=36.5g- 
HCl=49g.  H2  SO4.  They  are  made  by  dissolving  the  calculated  weight  of  reagent 
in  1000  cc.  of  water. 


CHAPTER  III 


THEORY  OF  SOLUTIONS  CONTINUED 
METATHESIS 

17.  Solvents. — The  solvents  generally  used  in  analysis  are  water, 
the  alkali  hydroxides  and  sulphides,  the  common  acids,  carbon  bisul- 
phide,  ether,   and  ethyl  (or  methyl)  alcohol.     The  solution  effected 
by  water  is  usually  physical;  but  it  may  be  accompanied  by  chemical 
change  as  seen  in  the  solution  of  alkali  oxides.     Alcohol,  ether,  and 
carbon  bisulphide  are  used  to  but  a  very  limited  extent  in  qualitative 
work,    and   chiefly   as   physical   solvents.     Consequently,    the    most 
important  chemical  solvents  are  aqueous  solutions  of  the  common  acids 
and  alkalis. 

18.  Chemical  activity. — It  has  been  ascertained  by  experiment 
that  substances,  even  those  with  which    we  associate  the  idea  of  the 
highest  chemical  activity,   are  incapable  of  reaction  except  in   the 
presence  of  some  agent  capable  of  producing  dissociation.     Thus,  dry 
chlorine  does  not  react  with  fused  sodium,  and  dry  phosphorus  does 
not  burn  in  dry  oxygen.     Water  is  the  most  powerful  dissociating 
fluid  known.     Since   electrolytes    manifest  their  greatest  activity  in 
very  dilute  solutions,  wherein  we  may  assume  dissociation  to  be  com- 
plete (§  14),  it  is  obvious  that  chemical  activity,  or  strength,  depends 
upon  degree  of  ionization,  and  chemical  reaction  occurs  between  the  ions. 

In  equivalent,  or  normal,  solution  (§  14),  it  is  shown  by  the  meas- 
urement of  their  electric  conductivities  that  neutral  salts  are  dissoci- 
ated to  the  extent  of  80  or  90  per  cent ;  the  strong  acids — HC1,  HBr, 
HI,  HNO3,  H2SO4,  HC1O3,  HC1O,,  and  the  thionic  acids,  to  about 
the  same  extent  ;  the  moderately  strong  acids  —  H3PO4,  H2SO3, 
HC2H3O2,  to  less  than  10  per  cent;  and  the  weak  acids— H2CO3,  H2S, 
HCN,  H3BO3,  H2SiO3,  to  less  than  1  per  cent;  the  strong  bases  — 
hydroxides  of  the  alkali  and  the  alkaline-earth  metals,  to  80  or  90  per 


QUALITATIVE    ANALYSIS  17 

cent;  the  moderately  strong  bases  —  NH^OH,  Mg(OH)2,  Ag2O 
(AgOH  does  not  occur  free),  to  less  than  10  per  cent;  hydroxides  of 
other  metals  are  weak  bases,  dissociating  to  less  than  1  per  cent. 

19.  Chemical  reactions.  —  It  appears  from  the  discussion  given  in 
the  preceding  section  that  chemical  reaction  only  takes  place  between 
ions.     How  this  occurs  may  be  illustrated  by  the  familiar  experiment 
of  making  common  salt,  NaCl.     The  solution  of  sodium  hydroxide  is 
electrolytically  dissociated  into  Na+  and  OH~.   The  solution  of  hydro- 
chloric acid  is  dissociated  into  H+  and  Cl~.     This  admits  of  readjust- 
ment according  to  the  law  of  electrical  attraction  by  which  the  Na 
and  Cl  ions  on  one  hand  and  the  H  and  OH  ions  on  the  other  are 
brought   together,  and   molecules  of  NaCl   and  H2O  result.     Since 
sodium  and  hydrogen  ions  are  both  electro-positive,  it  might  seem  that 
each  has  fully  as  good  a  chance  to  reunite  with  its  own  anion  as  with 
that  of  the  other,  unless  ionic  attractions  are  of  different  values;  but  we 
know  this  to  be  the  case,  as  otherwise  chemical  readjustment  could 
never  take  place. 

20.  fletathetical  ionization.  —  lonization    may    occur  otherwise 
than  by  electrolytic  dissociation.     There  are  three  cases  which  may 
be  illustrated  by  the  following  familiar  facts:  — 

1.  If  an  iron  spatula  be  dipped  into  a  solution  of  copper  sulphate, 
iron  goes  into  solution  and  copper  is  deposited,  — 

(Cu+++S0")  +  Fe=  (Fe+++S07-)-|-Cu. 

2.  A  piece  of  gold-leaf  readily  dissolves  in  chlorine-  water,  — 

Au+(Cl+Cl+Cl)=(Au++++Cl-+Cl--fCl-). 

3.  A  substance  in  solution  may  have  its  degree  of  valence  altered, 
as  when  ferrous  chloride  is  changed  to  ferric  chloride  by  treating  it 
with  nascent  chlorine,  — 


Reactions  may  be  met  with  which  fall  entirely  within  the  purview 
of  one  of  the  four  modes  of  ionization  (§§  12,  15;  and  the  preceding 
paragraph  of  this  section);  others  may  demand  two  or  more  for  their 
full  elucidation. 


18  QUALITATIVE    ANALYSIS 

21.  Chemical  equilibrium. — The  statement  concerning  chemical 
reaction  given  in  section  19  does  not  present  all  of  the  facts  involved. 
It  is  therein  assumed,  first,  that  the  solute,  sodium  hydroxide,  is  in 
its  solvent,  water,  wholly  ionized  —  NaOH»-*-Na+,  OH";  second; 
that  the  solute,  hydrochloric  acid,  is  in  its  solvent,  water,  also  wholly 
ionized — HC1^->H+,  Cl~;  and,  third,  that  the  products  of  metath- 
esis, common  salt  and  water,  are  in  the  common  solvent,  water, 
entirely  undissociated: — 

1.  Na+-fOH-+H++Cr^>[NaCl]  +  [H2O].  The  first  two 
assumptions  might  be  actually  attained  (§14);  but  the  third  assump- 
tion can  not  be  attained  in  practice  (§§15,  18).  Hence,  for  purposes 
of  general  illustration,  equation  1  must  be  replaced  by  the  following 
form : — 


2.  x[NaOH]*  +  yNa+  +  y  OH~  +  w[HCl]  +  zH+  +  zC 
v[NaCl]    +     v[H2O]    +    tNa+   +    tCr  +    sH+  -f  sOH".     This 
equation   indicates  the  facts,   namely,    that  in   a  solution   containing 
caustic  soda  and   hydrochloric  acid   some  molecules  of  the  reagents 
ionize,  these  ions  interact  to  produce  some  molecules  of  sodium  chlo- 
ride and  water,  certain  molecules  whereof  are  further  electrolyzed  into 
their  original  ions.      In  other  words,  the  reaction  is  incomplete  and  to 
some  extent  reversible.     So  that  equation  2  may  be  written  yet  more 
concisely  thus, — 

3.  [NaOH]  +  Na++OH-  -f  [HC1]  +  H+  -f  Cl~  ££  [NaCl]  + 
[H2O]  -f  Na+  -f  Cl~+  H+  -}-  OH".  It  will  be  noted  that  equation  3 
represents  two  reactions,  the  first  proceeding  from  left  to  right  and  the 
second  from  right  to  left.     These  reactions  are  not  occurring  alternately 
without   termination,   else  no  reaction  could  be  carried  to  a  definite 
stage  of  completion.      Instead,   the  reaction  towards  the  right  hand 
begins  at  a  maximum  rate  and  gradually  diminishes  in  velocity,  while 
simultaneously  the  reaction  towards  the  left  hand  begins  at  zero  and 
increases  in  velocity  until  NaCl  is  produced  at  precisely  the  same  rate 
at  which  it  is  being  ionized.      When  this  condition  is  attained,  chemi- 

*A  formula  thus  bracketed  indicates  an  undissociated  molecule. 


QUALITATIVE    ANALYSIS  19 

cal  equilibrium  results,  and  no  further  change  can  occur  in  the  solution 
so  long  as  temperature  and  pressure  are  constant. 

22.  Rate  of  reaction. — The  speed  of  reaction  is  expressed  by  the 
law  of  mass  action  as  follows:    When  substances  react  chemically  with 
one  another  the  rate  of  action  is  in  direct  ratio  to  the  active  masses  of 
the  reagents.     By    '  *  active  mass, ' '   we  understand    concentration  of 
ionic  mass  (§  18  and  §  23). 

23.  Concentration  of  a  solution. — The  distribution  of  a  reagent 
in  its  solvent  is  in  accordance  with  the  physical  laws  previously  set 
forth  (§§7,   14,   15).     The  number  of  gram-molecules*  of  a  given 
solute  present  in  1000  cc.  of  solvent  is  called  the  concentration.     In  a 
saturated  solution  (§8)  in  the  presence  of  some  undissolved  solute, 
there  is  a  state  of  equilibrium  between  the  dissolved  and  undissolved 
portions  of  the  solute,  and  also  between  the  dissociated  and  the  undis- 
sociated  portions  of  the  solute  in  solution.   Thus,  in  the  case  of  sodium 
hydroxide,    we  have  the    equilibrium    equation    [Na+]    X    [OH~] 
=cf  [NaOH] ,  wherein  the  bracketed  formulas  represent  the  respective 
concentrations.      But  for  a  given  solvent,  the  portion  undissociated  is 
of  constant  value;  therefore,  the  product  of  the  left-hand  members  of 
the  equation  is  constant,  which  is  to  say  that  in  a  saturated  solution  the 
product  of  the  ion- concentrations  is  always  of  the  same  value.     The 
product  obtained  by  multiplying  together  the  values  of  the  ion-con- 
centrations is  called  the  solubility -product. 

Since  the  concentration  of  the  ions  is  readily  measured  (§  14),  we 
may  easily  ascertain  what  relationship  subsists  between  the  solubility 
of  a  substance  (§9)  and  this  factor.  Experiment  shows  that  solubil- 
ity is  a  direct  function  of  the  solubility -product. 

If  the  equilibrium  equation  for  sodium  hydroxide  be  written  with 
concentration-factor  symbols,  it  takes  the  form  k2k3=rkr  which  is  per- 
fectly general  for  all  electrolytes  of  the  same  class.  Experiment 

*3ee  §11,  foot-note. 

fThis  c  is  the  constant  of  dissociation  for  the  given  solvent.    See  \  11,  last  para- 
graph, and  2 12,  first  paragraph. 


20  QUALITATIVE    ANALYSIS 

shows  that  over  a  wide  range  of  dilution  the  quotient  of  k2k3-f-k1  is 
practically  constant  in  value,  or 

yL°-c 

k, 

This  may  be  read  —  The  product  of  the  ion-concentrations  is  directly 
proportional  to  the  concentration  of  the  undissociated  solute.  But 
this  is  only  the  law  of  mass-action  in  other  words.  The  above  mathe- 
matical formulization  of  the  law  is  found  convenient  in  discussing 
reactions. 

24.  Change  of  equilibrium. — It  is  obvious  on  inspection  of  the 
above  equation  that  a  change  in  one  of  the  factors  of  a  reaction 
destroys  the  equilibrium  then  existing,  and  necessitates  a  correspond- 
ing change  in  the  other  factors  before  a  new  state  of  equilibrium  can 
ensue.  Two  cases  may  arise: — 

a.  The  concentration  of  one  of  the  ions  is  increased  in  a  saturated 
solution.  Since  the  value  of  the  product,  k2k3,  cannot  increase,  an 
increase  of  one  kind  of  ion  must  be  accompanied  by  corresponding 
decrease  of  the  other  whereby  kx  is  produced  and  precipitated.  To 
illustrate:  Let  sodium  chloride  be  the  solute  in  question.  The  equi- 
librium equation  for  its  saturated  solution  is  Na+  X  Cl~=  c  [NaCl] . 
If  now  the  chlorine  ions  be  increased  in  any  way,  as  by  adding  to  the 
solution  HC1,  or  KC1,  or  chlorine  gas,  etc. ,  the  solubility-product  is  ex- 
ceeded, rendering  the  quotient,  -~=C-\-x,  too  large  for  equilibrium, 

KI 
and  necessitating  the  deposition  of  the  excess,  x.     It  therefore  appears 

that  the  solubility  of  an  electrolyte  is  decreased  by  the  presence  of 
another  with  which  it  has  an  ion  in  common.  For  this  reason  we  use 
in  practice  "an  excess"  of  the  reagent. 

-  b.  The  concentration  of  one  of  the  ions  is  diminished  in  saturated 
solution.  If  the  concentration  of  either  kind  of  ion  be  lessened,  as  by 
taking  up  a  simple  ion  to  form  a  more  complex  ion  (§  20)  or  an 
undissociated  molecule,  the  value  of  the  quotient  in  the  equilibrium 
equation  becomes  too  small  by  x,  necessitating  more  of  the  solid  solute 
dissolving  and  ionizing  in  order  to  restore  equilibrium.  Wherefore, 


QUALITATIVE    ANALYSIS  21 

the  solubility  of  an  electrolyte  is  increased  by  decreasing  the  concentra- 
tion of  one  of  its  ions.  Illustrations:  (1).  A  slightly  dissociated  base 
(§  18)  dissolves  more  readily  in  acid  than  in  water  solution.  From 
the  fact  that  water  is  a  non-conductor  of  the  electric  current  (§  12) 
it  follows  that  it  ionizes  (H2O  m-*-  H+4~OH~)  to  but  an  infinitesimal 
extent.  Therefore  when  a  strongly  ionized  acid,  as  HC1,  and  a 
slightly  ionized  base,  as  Mg(OH)2,  are  brought  together  the  H+  of 
the  acid  at  once  unites  with  the  OH~  of  the  base,  forming  undisso- 
ciated  H2O.  This  diminishes  the  value  of  the  solubility-product- 
factor  OH~,  necessitating  more  of  the  solid  solute,  Mg(OH)2, 
dissolving.  (2).  The  salt  of  a  weak*  acid  dissolves  in  a  stronger 
acid.  Thus,  NaC2H3O2  dissolves  in  HC1.  Here,  the  C2H3Q-  ion 
unites  with  the  H+  of  the  acid,  producing  slightly  dissociated 
HC2H3O2.  (3).  The  formation  of  a  complex  ion  by  the  metathesis 
facilitates  solution.  Thus,  ferrous  hydroxide,  which  is  but  slightly 
soluble  (Fe++,  OH~,  OH~),  dissolves  readily  in  potassium  cyanide 
(K+,  CN~)  because  the  latter  anion  reacts  with  Fe++  to  form  the 
complex  ion  Fe(CN)~ .(4)  The  formation  of  an  undissociated  mole- 
cule facilitates  solution.  Thus,  lead  hydroxide,  which  is  but  slightly 
soluble  in  water,  readily  dissolves  in  sulphuric  acid.  Here,  the  SO" 
unites  with  the  Pb++,  forming  undissociated  PbSO4  which  is  precipi- 
tated and  so  removed  from  participation  in  the  reaction,  thereby 
necessitating  a  continuous  production  of  OH~  in  the  attempt  to  main- 
tain an  equilibrium. 

Since  the  chemical  activity  of  an  electrolyte  is  in  direct  proportion 
to  the  degree  of  ionization  (§§  18,  22),  it  follows  as  a  corollary  to 
proposition  a  that  weak  acids  and  bases  are  rendered  yet  weaker  by  the 
presence  of  their  neutral  salts.  This  is  taken  advantage  of  in  order  to 
expel  a  volatile  product  of  reaction  by  heat,  or  to  render  it  inactive  in 
the  presence  of  other  reagents.  Thus,  a  solution  of  ammonium 
hydroxide  is  but  slightly  dissociated  into  NH+,  OH~.  If  now  a 
strong  base,  as  NaOH,  be  added,  the  increase  of  OH  ions  necessitates 
the  formation  of  undissociated  NH4OH,  which,  being  volatile,  is 

*See  1 18. 


22  QUALITATIVE    ANALYSIS 

readily  driven  from  the  solution  by  heat.  Again,  the  addition  of 
NH4C1  (NH+,  C1-)  to  a  solution  of  MgCl  (Mg++,  Cl~,  Cl~)  causes  a 
regression  of  the  latter  ions  into  undissociated  MgCl2  and  so  prevents 
the  precipitation  of  MgCO3  by  NH4CO3. 

25.  Hydrolysis. — The  action  of  water  whereby  solutions  of  certain 
neutral  salts  show   "an  acid"   or  "an   alkaline  reaction"  is  called 
hydrolysis.     For  example,  Na2CO3  reacts  alkaline,  and    Sn012  reacts 
acid.     This  is  due  to  the  regression  of  ions  into  undissociated   mole- 
cules (§  24,  b,  4).     In  the  solution  of  sodium  carbonate,  we  have  the 
ion-factors  Na+,  Na+,  CO3~ ;     H+,  OH".     The  products  are  H2CO3, 
but  slightly  ionized,    and    NaOH,    strongly  ionized    (§  19),    conse- 
quently .'i-' i  alkaline  reaction.     In  the  stannous  chloride  solution,  we 
have  the  ion-factors  Sn++,   Cl~,   Cl~;  H+,  OH~.     The  products  are 
Sn(OH)2,   very  slightly  ionized,    and  HC1,    very  strongly  ionized  ; 
hence,    an  acid  reaction.     In  the  case  of  some  metalloids,  as  tin,  the 
regression  into  undissociated  molecules  may  go  so  far  as  to  lead  to 
their  precipitation  in  water  solution  as  hydroxides  and  basic  salts. 

26.  Completion   of   reactions.  —  It    appears    (§21)    that   all 
reactions  are  to  some  extent  reversible.     However,  for  analytical  pur- 
poses, we  select  those  that  may  be  rendered  practically  complete  under 
easily  attainable  conditions.     This  may  lead  us  to  the  precipitation  of 
a    salt,    the    formation  of  a  soluble   complex   (either   molecular    or 
ionic)  with  a  markedly  individual  color,  or  to  the  formation  of  undis- 
sociated volatile  molecules  of  characteristic  odor. 

Before  a  substance  can  be  precipitated,  the  solution  must  be 
rendered  saturated  with  its  undissociated  molecules  (§24,  a).  By 
way  of  illustration,  we  will  now  complete  the  discussion  of  the  reaction 
by  which  we  obtain  sodium  chloride  (§§  19,  21).  Chemical  equilib- 
rium having  been  established,  we  have  to  saturate  the  solution  for 
[NaCl] .  While  this  may  be  done  by  the  methods  indicated  in  §  24, 
a,  in  practice  we  resort  to  another  method  for  increasing  the  concen- 
tration of  the  [NaCl],  namely,  removal  of  the  other  association 
product,  [H2O].  This  is  done  by  evaporation.  The  operation, 
therefore,  transpires  thus:  By  solution,  there  results  [NaOH],  Na+, 


QUALITATIVE    ANALYSIS  23 

OH-,  [HC1],  H+,  C1-;  some  ions  react,  forming  [NaCl]  and  [H2O], 
necessitating  further  ionization  of  the  reagents  to  restore  the  equilib- 
rium thereby  lost;  and  so  proceeding  towards  saturation.  The  exper- 
imenter meanwhile  hastens  the  outcome  by  evaporating  some  [H2O] , 
thereby  reducing  the  ionizing  power  of  the  solvent-mass  and  conse- 
quently increasing  the  relative  number  of  undissociated  molecules  of 
sodium  chloride.  The  chemical  process  of  reaction  and  the  physical 
process  of  evaporation,  proceeding  from  opposite  directions,  thus  con- 
verge along  lines  which  meet  at  the  saturation-point.  The  moment 
this  is  passed  some  precipitation  occurs.  Finally,  the  reagents  having 
been  completely  ionized,  and  no  more  water  being  present  to  occasion 
reverse  ionization  of  NaCl,  nothing  but  the  latter  substance  remains. 
The  evaporation  method  for  effecting  precipitation  is  not  much  used 
in  qualitative  work.  Obviously  that  result  is  more  expeditiously 
reached  by  producing  metathetically  a  new  molecule  whose  solubility- 
product  is  slight  (as  PbSO4,  §  24,  6,  4).  But  in  reactions  whereby 
one  product  may  eventuate  as  a  solid  and  the  other  as  a  liquid  (as  in 
the  case  of  NaCl),  the  method  is  well  adapted  to  securing  all  of  the 
solid  product.  It  is  also  utilized  in  separating  solid-solutes  having 
differing  solubility  products.  The  latter  operation  is  called  fractional 
crystallization. 


CHAPTER    IV 


THE  ANALYTIC  GROUPS 

27.  The  course  outlined. — The  number  of  bodies  is  infinitely 
great,  and  an  infinity  of  time  would  be  required  by  an  intelligent 
being  merely  to  pass  them  in  review.  And  were  our  knowledge  of 
them  derived  solely  from  their  particular  properties,  an  eternity  would 
be  needed  in  order  to  acquire  "a  working  knowledge' '  of  the  world  about 
us.  Fortunately,  we  have  a  more  expeditious  way  for  acquiring  such 
information  (§  3,  last  paragraph).  The  student  is  already  aware 
from  the  study  of  General  Chemistry  that  all  aggregrates  of  matter 
fall  into  two  classes* — single  bodies  (elements),  and  compounds  (com- 
binations of  two  or  more  elements) ;  and  that  compounds  are  naturally 
divided  into  binaries  (as  oxides,  sulphides,  nitrides,  etc.),  adds, 
bases,  and  salts.  To  the  chemist,  therefore,  any  given  substance  is 
either  an  element,  an  oxide  (sulphide,  etc.),  an  acid,  a  base,  a  salt, 
or  some  combination  of  bodies  belonging  to  these  classes. 

It  has  been  stated  previously  (§2)  that  this  treatise  is  a  guide  to 
the  qualitative  analysis  of  the  commonly  occuring  substances^  merely. 
The  discussion  will,  therefore,  be  confined  to  the  elements  gold,  plat- 
inum, silver,  lead,  mercury,  copper,  cadmium,  bismuth,  arsenic,  anti- 
mony, tin,  iron,  chromium,  aluminum,  cobalt,  nickel,  manganese,  zinc, 
barium,  strontium,  calcium,  magnesium,  lithium,  sodium,  potassium, 
the  radical  ammonium,  their  oxides  and  sulphides,  and  their  salts 
with  the  following  acids:  HF,  HC1,  HC1O3,  HBr,  HI,  HNO3,  H2S, 
H2SOV  H2SO4,  H2CrO4,  H3BO3,  H3PO4,  H2SiO3,  H(CN),  H4Fe(CN)6, 
H3Fe(CN)6,  H2C03,  HC2H3O2,  H2C2O4,  H2C4H4O6,  H8C.HBOrt 

*Mechanical  aggregates,  or  mixtures,  need  no  discussion  from  the  chemical  point 
of  view. 

fit  may  be  stated  for  the  benefit  of  the  student  who  has  no  acquaintance  with 
Organic  Chemistry  that  the  last  four  formulas  are  those  of  acetic,  oxalic,  tarlaric,  and 
citric  acids,  respectively. 


QUALITATIVE    ANALYSIS  25 

28.  The  analytic  groups. — In  determining  a  substance  by  anal- 
ysis, we  have  to  discover  whether  it  is  purely  elemental  or  compound. 
If  the  latter,  its  class  must  be  ascertained ;  and  if  it  prove  to  be  a  salt, 
both  the  base  and  the  acid  involved  in  its  formation  must  be  identified. 
Considering  only  the  twenty-six  basic  elements  and  the  twenty-one 
acids  mentioned  above,  it  will  be  seen  that  the  number  of  their  possi- 
ble combinations  is  quite  large;  and  bearing  in  mind  that  a  substance 
presented  for  analysis  may  be  a  mixture  of  several  compounds,  we 
would  be  overwhelmed  by  the  difficulty  of  the  problem  were  there  no 
means  of  simplifying  it.  But  fortunately  two  ways  to  this  end  have 
been  found.  The  first  lies  in  the  fact  that  all  reactions  are  ionic  (§18) 
and,  consequently,  all  compounds  formed  in  the  course  of  analysis  are 
combinations  of  ions.  Inasmuch  as  there  are  but  few  kinds  of  ions 
(§  13),  the  identifying-properties  to  be  learned  are  correspondingly 
limited.  The  second  means  lies  in  the  fact  that  the  basic  ions  may 
be  readily  separated  by  certain  reagents  intone  groups,  from  which 
each  member  may  in  turn  be  isolated  and  then  identified.  The  acidic 
ions  are  as  readily  identified  as  the  basic  ones;  but  no  scientific  grouping 
having  yet  been  found  for  them,  they  are  divided  empirically  into 
four  groups. 

THE  BASIC-ION  GROUPS 

GROUP  I. — This  embraces  the  elements  that  form  insoluble  chlorides 
in  an  acidified  solution:  Ag,  !Hg,  [Pb]*.  The  group-reagent  is 
HClf;  hence,  this  group  may  also  be  called  the  hydrochloric-acid 
group. 

GROUP  II. — This  embraces  the  elements  whose  sulphides  are  insolu- 
ble in  cold  dilute-acid  solution:  "Hg,  [Pb]  ;  Cu,  Cd,  Bi;  As,  Sn,  Sb; 
Au,  Pt.  The  group-reagent  is  H2S;  hence,  this  group  may  also  be 
called  the  sulphuretted- hydrogen  group. 

GROUP  III. — This  embraces  the  elements  that  form  insoluble  hy- 
droxides or  sulphides  in  ammonium  hydroxide  solution:  Fe,  Al,  Cr; 

*£jead  chloride  is  somewhat  soluble,  and  may  appear  in  the  next  group  also. 

fThe  formula  for  an  acid  if  printed  in  ordinary  type  will  be  read  dilute',  if  in 
heavy  type,  concentrated. 


26  QUALITATIVE    ANALYSIS 

Co,  Ni;  Mn,  Zn.  The  group-reagent  is  (NH4)2S;  hence,  this  group 
may  also  be  called  the  ammonium-sulphide  group. 

GROUP  IV. — This  embraces  the  elements  that  form  insoluble  carbo- 
nates in  the  presence  of  NH4C1:  Ba,  Sr,  Ca.  The  group-reagent  is 
(NH4)2CO3;  hence,  this  group  may  also  be  called  the  ammonium- 
carbonate  group. 

GROUP  V. — This  embraces  the  elements  whose  chlorides,  sulphides, 
and  carbonates  are  soluble  in  the  presence  of  NH4C1:  Mg,  Li,  Na,  K, 
[NHJ .  It  may  also  be  called  the  alkali-metal  group. 

THE  ACID-ION  GROUPS 

GROUP  A. — The  organic-acid  group  embraces  the  following  acids: 
HC2H302,  H2C204,  H2C4H406,  H3C6H5O7. 

GROUP  B.  —  The  barium-hydroxide  group  embraces  the  following 
acids:  H2SO4,  H2SO3,  H2CrO4,  H3BO3,  H2C2O4. 

GROUP  C. — The  silver-nitrate  group  embraces  the  following  acids: 
HI,  HBr,  H(CN),  H4  Fe(CN)6,  H3  Fe(CN)6,  HC1. 

GROUP  D.  —  The  original- substance  group  embraces  the  following 
acids:  HN03,  HC1O3,  H3P04,  HF,  H2CO3,  H2S,  H2SiO3. 


CHAPTER    V 

IDENTIFICATION  OF  THE  BASIC  IONS 

29.  Group  I. — Members  of  this  group  may  come  to  the  analyst  as 
the  pure  metal,  as  an  alloy,  as  a  salt,  and  (with  the  exception  of 
silver)  as  an  oxide.  Sulphides  and  minerals — other  than  the  oxides 
and  the  hydroxides — will  be  regarded  in  this  book  as  salts.* 

SILVER  forms  the  colorless  ions  Ag+,  Ag(NH3)  +  ;  Ag~,  Ag(CN)JT. 

Identification:]  1.  With  chlorine,  silver  is  precipitated  as  AgCl, 
white  and  curdy,  soluble  in  NH4OH:—  Ag+,  CF-f3NH^= 
(NH3)3  Ag  Cl.§  The  solubility  of  the  reaction  product  is  due  to  its 
ready  ionization,— (NH3)3AgCl — >Ag(NH3)+,  NH+,  CF.  From  this 
solution,  AgCl  is  again  precipitated  upon  acidification  by  reason  of  the 
hydrogen  ions  of  the  acid  combining  with  NH3+,  to  form  NH+,  which 
unites  with  OH~  to  produce  [NH4OH],  thereby  increasing  the  con- 
centration of  Ag+  and  CF  (§26). 

2.  With  CrO4  ,  silver  is  precipitated  as  Ag2CrO4,  brick -red  and 
crystalline,  soluble  in  HNO3  and  in  NH4OH: 

Ag+,  00;-+2H+,  NOiT=Ag+,  ISTO~+  [H,CrOJ  ; 

Ag+,  CrO:-+2NH~1[+2NH4+=2NH4+,  CrO"+Ag(NH8)- 

MERCURY  (THg)  forms  the  colorless  ions  Hg+  and  Hg^H^. 

*For  a  description  of  the  elements  and  their  compounds,  the  student  is  referred  to 
a  standard  work  on  General  Chemistry. 

fAny  insoluble,  markedly  colored,  or  strikingly  odorific  compound  of  an  element 
may  serve  in  general  to  identify  it;  but  in  practice  we  confine  our  attention  to  a  few 
of  the  most  notable  ones. 

{Throughout  this  work,  a  line  under  a  formula  indicates  the  formation  of  a  solu- 
ble product;  a  brace  below  Indicates  a  precipitate;  and  a  brace  above  means  the 
evolution  of  a  gas. 

git  is  doubtful  if  the  compound  NH4OH  actually  exists.  A  solution  of  NH3  in 
water  is  very  slighily  ionized  into  NH4+,  OH  ,  due  to  the  fact  that  NH3+  being  less 
electro-positive  than  H+  a  complex  ion  NH3H+  is  formed.  A  solution  of  the  gas. 
therefore,  holds  the  ions  NH3+,  NH4+,  H+,  OH'  and  undissociated  [NH4OH]  and 

pf  the  ammonia  Ion  is  less  electro-positive  than  another  positive  ion  preient,  it 
may  behave  as  an  anion  (see  last  foot-note).  This  is  a  general  rule  for  ions. 


28  QUALITATIVE    ANALYSIS 

Identification:    1.    With    chlorine,    ous-mercury  is  precipitated   as 
HgCl,   white  and  curdy,   soluble  in  aqua  regia,   but  transposed  into 
mercurous  chloramid,  Hg2NH2Cl,  black  and  insoluble,  byNH4OH:— 
(Hg+,  Cl-)+Cl=HgCl2^Hg++,  2C1-(§21,2); 


LEAD  forms  the  colorless  ions  Pb++,  PbO2~,  and  PbOC2H3O2~. 

Identification:  1.  With  chlorine,  lead  is  precipitated  as  PbCl,  , 
white  and  curdy,  slightly  soluble  in  cold  water  but  very  soluble  in  hot 
water. 

2.  From  the  above-mentioned  aqueous  solution,  lead  is  reprecipitated 
by  CrO"  as  PbCrO4,  bright  yellow  and  flocculent-granular:  — 
Pb+++2Cl-=[PbCl2]; 

Pb++,  2C1-+2H+,  CrO"  =2H+,  2C1~+  [PbCrOJ. 

30.  Group  II.  —  Members  of  this  group  may  come  to  the  analyst  in 
the  elemental  state,  as  an  alloy,  as  a  salt,  and  (excepting  gold  and 
platinum)  as  oxides. 

LEAD  may  appear  in  this  group  in  the  course  of  analysis  after  the 
removal  of  members  of  the  Hydrochloric-acid  Group  on  account  of 
the  solubility  of  PbCl2.  It  must  be  removed  before  proceeding  to 
analyse  for  members  of  the  group  proper.  This  is  done  by  taking 
advantage  of  the  fact  that  with  SO"  PbSO4  is  precipitated,  white, 
granular,  and  insoluble  in  the  dilute-acid  solution  being.  used  at  the 
time.  In  this  form,  it  is  readily  removed  by  filtration. 

MERCURY  ("Hg)  forms  the  colorless  ions  Hg++  and  HgNH2++. 

Identification:  1.  With  S  (H2S),  ic-mercury  is  precipitated  as 
HgS,  black  and  granular,  or  whitish-yellow  to  brown  if  the  reagent 
be  not  in  excess,  insoluble  in  HNOs  but  soluble  in  aqua  regia.  If 
ous-mercury  be  present  during  the  treatment  with  H2S,  it  is  trans- 
posed into  HgS  and  Hg  (§20,2).  The  solution  in  aqua  regia  is  re- 
duced by  SnCl2  to  HgCl  or,  if  the  reagent  be  in  excess,  to  HgCl  and 
Hg,  this  mixture  appearing  brown  to  black,  owing  to  the  formation 
ofHg20:— 


QUALITATIVE    ANALYSIS  29 

[HgCl2]  ; 
2Hg++,  4Cl-,-fSn++,  2Cl--=Sn++++,  4Cl-+[2HgCl],   and 

Hg++,  2Cl--fSn++,  2Cl-=Sn++++,  4Cl~-f  [Hg]. 

COPPER  when  univalent  (:Cu)  forms  the  ions  Cu+,  and  Cu(CISr)7, 
colorless;  and  when  bivalent  (HCu),  the  ions  Cu++,  blue,  and  Cu- 
(NH3)4++,  intensely  blue. 

Identification:  1.  Copper  is  precipitated  by  S  in  slightly-acid 
solution  as  CuS,  black,  flocculent-granular,  slightly  soluble  in 
(NH4)2Sx,  soluble  in  HNOs  When  this  solution  is  diluted  and 
treated  with  NH4OH  to  excess,  an  intensely-blue  color  appears;  this 
is  due  to  the  formation  of  Cu(NH3)^"+:  — 

Cu++,  2NO8-+  (4NH3  )=Cu(NH3)4++,  2NQ-. 

2.   The  above-mentioned  color  is  discharged  by  K(CN):  — 

K+,Cu(CN)-. 


3.  Potassium  ferrocyanide,  K4Fe(CN)6,  precipitates  from  ammoni- 
acal  solution,  even  when  too  dilute  to  allow  the  blue  color  of  the 
Cu(KE3)4  ions  to  be  perceived,  Cu2Fe(CN)6,  reddish  brown:  — 

2Cu(NH3)4+++Fe(CN)6—    =8NH3+  [Cu,Fe(CN)fl] 


CADMIUM    forms   the   colorless   ions  Cd++,    Cd(CN)4~~,  and  Cd- 


Identification:  1.  With  S  ,  cadmium  is  precipitated  from  slightly- 
acid  solution  as  CdS,  bright-yellow  with  a  faint  suggestion  of  green, 
flocculent,  soluble  in  strong  solutions  of  the  halids  and  in  hot  HNOs, 
insoluble  in  K(CN). 

2.  From  a  solution  of  one  of  its  salts  (as  Cd(NO3)2),  NH/DH 
precipitates  Cd(OH)2,  white,  flocculent,  and  soluble  in  excess  of  the 
reagent.  Similarly,  K(CN)  precipitates  Cd(GN)2,  white,  dissolving 
in  excess  of  reagent.  From  these  solutions,  CdS  is  again  precipitated 
by  (NHJ2S  or  H2S:- 

Cd+++x(NH3)=Cd(NH3)x++; 

+,  Cd(CN)4~; 


Cd(NH3)x+++2NH4+,  S~=xNH8,  2NH4++[CdS]. 


30  QUALITATIVE    ANALYSIS 

BISMUTH  forms  the  colorless  ions  Bi+++  and  BiO+,  the  latter  rather 
strongly  electropositive  and  the  former  but  slightly  so.  Hence,  salts 
of  the  trivalent  ion  readily  hydrolyze  (§25)  to  basic  bismuthyl  salts. 

Identification:  1.  With  S  in  dilute  acid  solution,  bismuth  is 
precipitated  as  Bi2S3,  black,  flocculent,  insoluble  in  (NHJ2  Sx>  solu- 
ble in  hot  HNO3 

2.  From  a  solution  of  one  of  its  salts  (as  Bi(NO3)3),  NH4OH  pre- 
cipitates Bi(OH)3,  white,  flocculent,  insoluble  in  excess  of  the  reagent 
but  soluble  in  dilute  acid. 

3.  Upon  forming  BiCl3  as  just  indicated  in  2,  and  allowing  it  to  drop 
slowly  into  cold  water,  BiOCl  white  and  insoluble  is  formed  by  hydrol- 
ysis.     The  precipitation  is  facilitated  by  previous  solution  of  NaCl  in 
the  water  (§  24,  a). 

4.  A  stannite    (as  Sn(ONa)2)    reduces  the  hydroxide  to  the  ele- 
mental state,  which  will  here  show  black:  — 

Bi+++,  3OH-+Sn++,  2ONa~-f  H+,  OH-+  H+= 
Bi  -f  [Sn(OH)J  -f  2NaOH—  >2Na+,  OH~. 

ARSENIC  forms  the  colorless  ions  As  ,  As  ,  AsO  , 

AsO^  ,  .and  the  colored  (yellow)  ions  As  S^  ,  As  S^  . 

Detection  :  1.  In  slightly-acid  solution,  H2S  reduces  ic-ions  (if 
present)  to  ous-ions  which  are  precipitated  by  S  as  As2S3,  lemon- 
yellow,  flocculent,  soluble  in  (NH4)2Sx  (by  reason  of  the  formation  of 
).*  From  this  solution  As2S3  is  again  thrown  down  upon 


acidification,  soluble  in  (NH4)2CO3  and  in  aqua  regia.  In  the  former 
case,  the  ions  AsSg  are  again  formed,  so  that  As2S3  reappears  upon 
acidification;  in  the  latter,  As  and  As"  "  ions  are  produced. 
2.  The  aqua  regia  solution  diluted  and  treated  with  zinc  forms 
arsine,  AsH3,  in  which  the  presence  of  arsenic  may  be  shown  by  its 
reducing  effect  on  a  solution  of  silver  nitrate:  — 
As  —  ,  3H+  +  2Ag+,  2NO~  +  H+,  OH-+  20"  =2Ag+  [H3AsO3] 

-f  2HNO3.     The  reduced  silver  forms  a  black  precipitate. 

*Tln  polysulphide  (NH4)3  As  S3  ionizes  thus—  3NH4t,  As  S3~~;  and  so  .for  the 
other  polysulphides. 


QUALITATIVE    ANALYSIS  31 

ANTIMONY  forms  the  colorless  ions  Sb+++,  SbO+,  SbO^T,  SbOg", 
SbO8—  ,  SbO™-  ,  SbOC4H,Obr  and  the  colored  (reddish)  ions 
SbS8—  ,  SbS4—  . 

Detection:  1.  In  slightly-acid  solution,  H2S  reduces  ic-ions,  (if 
present)  to  ous-ions  which  are  precipitated  by  S  as  Sb2  S8,  orange- 
red,  flocculent,  soluble  in  (NH4)2Sx  (owing  to  the  formation  of  Sb- 
&~  )*.  From  this  solution,  Sb2S3  is  again  thrown  down  upon  acid- 
ification, insoluble  in  (NH4)2CO3,  but  soluble  in  aqua  regia,  owing  to 
the  formation  of  the  ions  Sb+++  and  (possibly)  Sb++"  f+. 

2.  The  aqua  regia  solution  diluted  and  treated  with  zinc  yields 
SbH3  which  is  decomposed  to  some  extent  in  the  generator  on  account 
of  the  relatively  weakly-electropositive  character  of  antimony  in  com- 
parison with  zinc  (§  20,1):  — 

(Sb+++,  2Cl-)  +  Zn=(Zn++,  2Cl-)+Sb. 

This  precipitate  shows  black;  if  relatively  small  in  amount,  it  may 
all  redissolve,  react  with  the  nascent  hydrogen  and  pass  out  of  the 
solution  as  SbH3  with  the  portion  that  was  not  decomposed.  The 
gas  reduces  a  solution  of  silver  nitrate: 

Sb—  ,  3H^+3Ag+,  3NO-=[Ag3Sb]+3H+,  3NQ-. 

Inasmuch  as  argentic  stibnide  is  black,  this  test  can  only  be  made 
in  the  presence  of  arsenic  by  so  diluting  the  aqua  regia  that  the  evolution 
of  hydrogen  is  very  slow.  In  this  case,  practically  all  the  antimony 
is  reduced  in  the  generator.  The  contents  of  the  generator  are  filtered, 
dissolved  in  HCI,  diluted,  and  treated  with  H2S.  Antimony  is 
precipitated  as  SbS3.  If  arsenic  has  been  removed  before  applying 
the  stibine  test  and  the  silver  nitrate  solution  shows  a  black  precipitate, 
this  is  due  to  the  formation  of  Ag3  Sb.  It  may  be  confirmed  by 
filtering,  dissolving  the  precipitate  in  HCI,  diluting  with  an  equal 
volume  of  water  (which  will  precipitate  AgCl),  filtering,  and  treating 
the  filtrate  with  H2S. 

TIN  forms  the  colorless  ions  Sn++++,  Sn++,  Sn  ----  ,  Sn(OH)-, 
Sn(OH)2++,  SnOH+++SnO2--,  SnO8~,  and  the  colored  ions  SnSi~ 
(  brown), 


*  ride  arsenic. 


32  QUALITATIVE    ANALYSIS 

Identification:  1.  In  slightly-acid  solution,  S  precipitates  oils-tin 
as  SnS,  dark-brown,  flocculent;  but  ic-tin  is  precipitated  very  slowly 
if  cold  and  slowly  if  hot,  as  SnS2,  white  so  long  as  the  salt  is  in  excess, 
then  yellow  and  flocculent.  These  sulphides  are  soluble  in  (NH4)2- 
Sx,  owing  to  the  formation  of  SnS".*  From  this  solution,  SnS 
and  SnS2  are  again  precipitated  upon  acidification,  insoluble  in 
(NH4)2CO3,  soluble  in  aqua  regia,  owing  to  the  formation  of  Sn+4++. 

2.  The     aqua    regia    solution    diluted    and    treated    with    zinc 
precipitates    tin,     gray    and    spongy,    on    the     zinc    (§20,1).     To 
confirm  the    test,    filter,   dissolve    the  residue  (freed  from    obvious 
fragments  of  zinc)  in  HC1,   and  add  the  solution  (2  or  3  cc.)  to  an 
equal  volume  of  HgCl2.     A  white   (HgCl)   or  black  to  gray  (Hg, 
HgCl)  precipitate  shows  tin. 

3.  If  antimony  is  present,  it  must  be  removed  before  applying  test 
2.     This  may  be  accomplished  by  effecting  the  reduction    with    a 
"voltaic  couple"  made  by  bending  a  strip  of  zinc  so  as  to  clasp  a 
strip  of  platinum  foil.     This  is  dropped  into  the  dilute  aqua  regia 
solution.     The  antimony  then  forms  a  black  deposit  adhering  to  the 
platinum,  while  the  tin  appears  on  the  zinc  but  does  not  adhere  to  it. 

The  foil  is  carefully  removed,  washed,  and  treated  with  a  little 
H2C4H4O6,  to  which  a  few  drops  of  HNO3  have  been  added.  The 
antimony  dissolves,  and  is  thrown  down  from  this  solution  by  H2  S. 
The  tin  is  now  tested  for  as  directed  in  2. 

4.  Tin  and  antimony  may  also  be  separated  by  taking  advantage  of 
the  different  behavior  of  their  ions  in  bromine-water.     The  mixed 
sulphides  precipitated  from  (NHJ2Sx  are  dissolved  in  the  least  pos- 
sible  amount  of  aqua  regia,  diluted  with   an  equal  volume  of  water, 
and  filtered   (=SbCl4  and  SnClJ.     The   filtrate   is  made  strongly 
alkaline  with  boiling  silica-free  KOH: — 

SbO,— -  -f3K+=K3SbO3; 
SnO"  -f-2K+:=K?Sn03. 

Bromine-water  is  now  added  to  excess.  Tin  is  precipitated  as 
/3H2SnO3  by  hydrolysis,  while  antimony  is  transposed  into  soluble 
complexes : — 

*(NH4)2  SnS3,  etc.,  being  formed  by  the  reaction. 


QUALITATIVE    ANALYSIS  33 

4Br-+4H+,    (4OH-)=4H+, 
[H2Sn03]  +[HfO]; 

3K+,  SbO8—  +  6Br- 

The  stannic  hydrate  is  white  and  flocculent.  When  this  is  removed 
by  filtration,  H2S  precipitates  Sb2S3  from  the  filtrate. 

GOLD  forms  the  ions  Au+,  Au+++,  Au  Cl^,  An  0,7,  Au  S~.  AuS2~. 

Identification:  1.  With  S  ,  gold  is  precipitated  in  slightly-acid 
solution  as  Au2S  and  Au2S2.  brown  to  black,  soluble  in  (NH4)2  Sx 
owing  to  the  formation  of  AuS^  and  AuS  ,  (  (NH4)8  Au2S3»-*- 
3NH4+,  Au  S~,  Au  S").  From  this  solution  the  sulphides  are  again 
thrown  down  upon  acidification. 

2.  Soluble  salts  of  gold  (as  H2AuCl4)  are  reduced  by  boiling  with 
(NH4)2  C4O4  and  H2C4O4  to  the  metallic  state  and  precipitated,  mixed 
with  some  ft  stannic  acid  (H2SnO3),  as  "the  purple  of  Cassius"  which 
shows  violet,  or  brown,  or  brownish-violet. 

PLATINUM  forms  the  ions  Pt++,  Pt+++,  PtCl",  Pt  S8~. 

Identification:  1.  With  S  ,  platinum  is  precipitated  in  slightly- 
acid  solution  as  PtS  or  PtS2,  black,  slowly  soluble  in  (NH4)2Sx,  and 
reprecipitated  upon  acidification. 

2.  Platinic  salts  are  reduced  by  SnCl2  to  ous-salts,  which  dissolve  in 
HC1  giving  a  deep-red  color  owing  to  the  formation  of  the  ion  PtClj["~. 
If  gold  is  present,  it  must  be  removed  (Gold,  2)  before  applying  this 
test. 

31.  Group  HI. — Members  of  this  group  may  come  to  the  analyst  as 
the  metal,  as  an  alloy,  as  a  salt,  and  as  an  oxide. 

IRON  forms  the  simple  ions  Fe++,  Fe+++,  FeO^4",  and  a  number 
of  complex  ions,  of  which  the  most  important  are  Fe(CN)""  "  and 
Fe  (CN)6— . 

Detection:  1.  Iron  is  precipitated  from  ic-salts  by  NH4OH  as  Fe- 
(OH)3,  which  is  not  soluble  in  Ba(OH)2,  nor  appreciably  so  in  NaOH. 
This  hydroxide  is  readily  transposed  by  S  ,  ((NH4)2S),  into  fer- 
rous sulphide,  FeS,  black,  and  readily  soluble  in  cold  dilute  acid. 

*The  solution  probably  contains  more  complex  compounds  of  antimony,  also. 


34  QUALITATIVE    ANALYSIS 

2.  The  ferrous  chloride  solution  (  1  )   is  easily  oxidized   by   boiling 
with  a  little  HNO3  to  FeCl3  (§  20,3).    From  this  solution,  NH4OH 
precipitates  Fe(OH)3,  dark-brownish-red,  gelatinous. 

3.  Solutions     of    ic-salts   give     with    S(CN)~,    (as  KS(CN)), 
'[Fe((CN)S)3],  deep-red  and  extremely  soluble  (§  *26).     &n  excess 
of  the  reagent  and  the  presence  of  an  acid  tend  to  intensify  the  color 
by  antagonizing  hydrolysis  (§  25). 

ALUMINUM  forms  the  simple  ions  Al+++,  AlO^,  A1CX  ,  and 
several  complex  ions  containing  the  hydroxyl  group. 

Detection:  1.  Aluminum  is  precipitated  by  OH~,  (NH4OH),  as 
A1(OH)3,  white,  gelatinous,  and  not  appreciably  soluble  in  an  excess 
of  this  reagent  (§  24,a).  It  is  not  transposed  by  S  ,  (NH4)2S,  into 
the  sulphide,  inasmuch  as  this  compound  is  at  once  decomposed  by 
hydrolysis  : 

+,  3S~  -f  6H+,  6OH-^[2A1(OH)3]+6H+,  3S~. 


2.  The  hydrate,  A1(OH)3,  dissolves  in  a  solution  of  a  stronger 
hydroxide  (as  Ba(OH)2  and  NaOH),  owing  to  the  fact  that  the  rel- 
atively greater  mass  of  more  strongly-basic  ions  (Ba++,  Na+)  rob  it 
of  OH~  (§  24,  b,  1).  Since  A1(OH)3  ionizes  to  but  a  trivial  degree, 
its  chemical  activity  is  slight  (§18).  From  this  basic-solution, 
A1(OH)3  is  again  precipitated  upon  neutralization,  or,  what  is  more 
easily  accomplished,  after  acidifying  and  then  adding  NH^OH  in  ex- 
cess (1). 

CHROMIUM  forms  the  ions  Cr++  (blue),  Cr+++  (violet),  Cr- 
(OH)2O~  (green),  CrO"  (yellow),  Cr2Or-  (red).  The  ous-ions 
are  readily  oxidized  to  the  ic-condition. 

Identification:  1.  Chromium  in  the  ic-condition  is  precipitated  by 
OH~,  NH4OH,  as  Cr(OH)3,  bluish-gray,  gelatinous-flocculent,  easily 
soluble  in  dilute  acid,  somewhat  soluble  in  NH4OH  by  reason  of  the 
formation  of  a  soluble  ammonia  complex,  and  rather  easily  soluble  in 
NaOH  or  KOH,  owing  to  the  formation  of  the  ion  Cr(OH)2O~, 
(e.  g.,  Cr(OH)2ONa).  Cr(OH3)  is  not  transposed  by  S~,  (NH4),8, 
on  account  of  hydrolysis. 


QUALITATIVE    ANALYSIS  35 

2.  Ic-compounds  upon  fusion  with  Na2CO3  yield  Na2CrO4,  yellow 
and  soluble. 

3.  A  clear  solution  of  a  chromate  (CrO4 )  upon  treatment  with 
lead  acetate  gives  a  precipitate  of  lead  chromate  (PbCrO4)  bright- 
yellow  and  flocculent  :  — 

2Na+,  CrO4—+Pb++,  2C1H8O2-=[PbCrOJ  +  2Na+,  2C2H3Q-. 

COBALT  forms  the  ions  Co++  (rose-red),  Co+++  (green),  Co- 
(NH3)n++  (blue),  Co  (NH3)3++,  Co(NH3)6+++,  Co(NO2)6 (red). 

Identification:  1.  With  OH~,  NH4OH,  ous-salts*  give  a  bluish 
basic  (NH3)  hydrate  which  changes  on  being  heated  to  Co(OH)2, 
pink  or  rose- red,  soluble  in  excess  of  the  reagent  and  of  NH4C1. 
Ammonium  sulphide  transposes  the  hydroxide,  or  a  salt  in  alkaline 
solution,  into  CoS,  black,  insoluble  in  HC1  (one  part  of  acid  to  10 
parts  water). 

2.  Oxides  of  cobalt  color  a  bead  of  borax  blue,  owing  to  the  forma- 
tion of  a  borate.  The  CoS  obtained  in  (1)  may  be  used  for  this 
purpose,  as  an  oxidizing  flame  ionizes  the  compound  and  burns  the 
elements  to  SO2  and  CoO  respectively. 

NICKEL  forms  the  ions  Ni++,  Ni(NH3)4++,  Ni(NH8)6++,  NiC4H4- 
0.~. 

Identification:  1.  With  OH~,  NH4OH,  ous-salts  give  Ni(OH)2, 
pale-green,  soluble  in  excess  of  the  reagent,  owing  to  the  formation  of 
basic  (NH3)  ions.  Ammonium  sulphide,  (NH4)2S,  transposes  these 
hydroxides  into  NiS,  black,  insoluble  in  HC1  (1:10),  but  soluble  in 
(NH4J2Sx,  probably  owing  to  the  formation  of  complex  basic  (NH3) 
ions.  This  solution  is  brown  to  violet  in  color.  Hence,  the  (JVJ5T4)2$ 
used  must  be  freshly  prepared  so  as  to  avoid  the  presence  in  it  of  any 
(NH4)2Sx;  and  the  previous  treatment  with  NH4OH  should  be  car- 
ried barely  beyond  neutrality  so  as  to  avoid  as  far  as  possible  the 
formation  of  nickel-ammonia  ions  and  ammonium  salts. 

2.   Oxides  of  nickel  color  a  borax-bead  violet  to  reddish-brown. 
The  sulphide  (1)  may  be  used  for  this  purpose.     The  presence  of 
cobalt  may  mask  this  test. 
*The  common  compounds  of  cobalt  are  of  the  ous-form. 


36  QUALITATIVE    ANALYSIS 

3.  Nickel  (sulphide)  (1)  may  be  separated  from  cobalt  (sulphide) 
by  taking  advantage  of  the  differing  solubilities  of  their  ic-hydroxides 
in  NH4OH.  To  accomplish  this,  the  mixed  sulphides  are  dissolved  in 
the  least  possible  amount  of  hot  HC1,  to  which  a  little  HNO3  has 
been  added,  boiled  till  the  liquid  is  nearly  all  evaporated,  and  NaOH 
then  added  to  strongly-alkaline  reaction.  The  hydroxides  at  first 
precipitated  by  the  last  reagent  dissolve  in  an  excess  of  it.  The 
solution  is  now  diluted  with  an  equal  volume  of  bromine  water  and 
again  boiled.  This  oxidizes  the  ous-hydroxides  to  the  ic-form,  and 
they  are  precipitated  as  a  black  powder.  This  is  allowed  to  settle, 
the  fluid  decanted,  and  the  precipitate  washed  by  decantation.  On 
boiling  the  precipitate  with  NH4OH,  Ni(OH)3  goes  into  solution.  It 
is  filtered  from  the  Co(OH)3,  and  nickel  is  then  precipitated  from  the 
filtrate  by  H2S.  This  precipitate  may  then  be  tested  further  as 
directed  in  2. 

MANGANESE  forms  the  ions  Mu++  (pink),  Mn+++,  MnO4  (green 
anion  of  manganates),  MnO4~  (purple  anion  of  permanganates). 

Detection:  I.  With  OH~,  NH4OH,  ous-salts  give  Mn(OH)2, 
white,  but  readily  oxidizing  upon  standing  in  solution  to  Mn(OH)3, 
which  is  brownish.  The  precipitation  is  incomplete,  owing  to  the 
fact  that  Mn(OH)2  is  more  strongly  basic  than  NH4OH(§  18);  and 
if  ammonia  salts  are  present,  no  precipitation  occurs  (§24,  b,  1)*. 
Ic-salts  with  OH~  yield  Mn  (OH)3  more  weakly  basic  than  Mn(OH)2. 
(NH4)2S  transposes  these  hydroxides  (and  salts  in  alkaline  solution) 
into  MnS,  flesh-pink,  flocculent-gelatinous,  easily  oxidized  (by  ex- 
posure to  the  air)  to  readily  soluble  MnSO4,  and  soluble  in  dilute 
acid. 

2.  Salts  of  manganese  (MnS  from  1  will  answer)  fused  on  a  plati- 
num foil  with  Na2CO3  and  KNO3  give  a  blue-green   ''melt,"  due  to 
the  formation  of  a  manganate  (MnO"). 

3.  Manganese  compounds  dissolved  in  HNO3  and  oxidized  by  one 
of  the  common  oxides  of  lead  yield  manganic  acid,  purple  (MnO^)  :  — 

Pb++-f  Mn++,  2NO8-+  H++4O~  -[HMn4]+Pb++,  2NCT. 


For  the  same  reason  Fe++  is  incompletely,  and  Mg*+  not  at  all,  precipitated  by 
NH4OH  in  presence  of  NH4C1. 


QUALITATIVE    ANALYSIS  37 

Reducing  agents  (az  zinc)  should  be  removed  before  applying  this 
test. 

ZINC  forms  the  colorless  ions  Zn++,  ZnO",  and  complex  ammonia- 
ions,  Zn(NH3)n++. 

Identification:  1.  With  OH~,  NH4OH,  zinc  salts  yield  Zn(OH)2, 
white,  gelatinous,  soluble  in  excels  of  the  reagent  owing  to  the  forma- 
tion of  zinc-ammonia  ions.  This  hydroxide  (and  the  complex  salts 
in  ammoniacal  solution)  are  transposed  by  (NH4)2S  (or  the  ion  S ) 
into  ZnS,  white  to  grayish -white,  and  easily  soluble  in  dilute  HC1. 

2.  Solutions  of  zinc  salts  (as  ZnCl2  from  1)  treated  with  Na2CO3 
yield  basic  zinc  carbonate  (Zn5(OH)6(CO3)2H2O)  by  hydrolysis  of  the 
carbonate  which  is  first  thrown  down.  The  precipitate  is  white  and 
insoluble  in  excess  of  the  reagent. 

32.  Group  IV, — Members  of  this  group  may  come  to  the  analyst 
as  a  salt  and  as  an  oxide. 

BARIUM  forms  the  colorless  ion  Ba++. 

Identification:  1.  With  CO",  (NH4)2CO3,  barium  is  precipitated 
from  alkaline  solution  as  BaCO3,  white  and  flocculent  but  becoming 
pulverulent-crystalline  on  standing,  easily  soluble  in  acids.  The 
presence  of  NH4C1  affects  this  precipitate  but  trivially,  since  Ba++  is 
more  strongly  basic  than  NH^  (§  18). 

2.  The  acetate  yields  with  CrO",  (K2CrO4),  BaCrO,,  light-yel- 
low, soluble  in  MCI ;  and  from  this  solution  SO",  (H2S04),  precipi- 
tates BaSO,,  white. 

STRONTIUM  forms  the  colorless  ion  Sr++. 

Identification:  1.  With  CO",  strontium  is  precipitated  from  al- 
kaline solution  as  SrCO3,  white  and  flocculent  but  becoming  pulveru- 
lent-crystalline on  standing,  not  affected  appreciably  by  NH4C1,  and 
easily  soluble  in  acids. 

2.  The  chloride  yields  SrSO,  with  SO",  white,  pulverulent-crys- 
talline. In  applying  this  test  barium  and  calcium  must  be  absent. 
The  former  may  be  removed  by  taking  advantage  of  the  relative  in- 
solubility of  its  chromate  in  acetic  acid  (2,  above),  and  the  latter  by 


38  QUALITATIVE    ANALYSIS 

effecting  the  sulphate  transposition  of  the  Sr++  by  means  of  CaSO4: — 

Sr+++Ca+++Ca++,  SO4~ :=[SrSOJ*+2Ca++. 

CALCIUM  forms  the  colorless  ion  Ca++. 

Identification:  1.  An  amraoniacal  solution  free  of  Ba++  and  Sr+4 
(see  Strontium  2,  above)  yields  with  C9O™,  (NH4)2C2O4,  calcium  oxa- 
late  (CaC2O4),  white  and  finely  pulverulent-crystalline. 

33.  Group  V. — Members  of  this  group  may  come  to  the  analyst  as 
the  metal  (magnesium  only),  as  an  alloy  (Mg,  Na,  K,  NH4 — the 
last  three  as  amalgams},  as  a  salt,  as  an  oxide  (Mg  only),  and  as 
hydroxides. 

MAGNESIUM  forms  the  colorless  ion  Mg++. 

Identification:  1.  Magnesium  is  arbitrarily  separated  from  its 
chemical  congeners  of  Group  IV  by  the  fact  that  the  carbonate  is  not 
precipitated  in  the  presence  of  NH4Cl.f  Thus  it  is  brought  by  the 
scheme  of  analysis  (Chap.  IV)  into  Group  V.  From  this  solution,  it 
is  precipitated  (by  adding  NH4OH,  to  strong  alkalinity,  and  then 
Na2HPO4)  as  MgNaPO,  and  MgNH4PO4,  white  and  finely  crystal- 
line. 

Before  applying  this  test,  members  of  Group  IV  must  be  completely 
removed,  as  they  yield  under  the  above  treatment  a  fiocculent  precip- 
itate that  may  obscure  that  of  magnesium. 

LITHIUM  forms  the  colorless  ion  Li+. 

Identification :  The  lithium  ion  colors  a  Bunsen  flame  £  bright-red, 
masked  by  sodium.  If  sodium  is  present,  the  flame  observed  through 
the  spectroscope  shows  a  scarlet  line  at  the  left  of  the  sodium  line. 

SODIUM  forms  the  colorless  ion  Na+. 

Identification :  The  sodium  ion  colors  the  flame  intensely  yellow. 
When  this  is  observed  through  the  spectroscope,  a  bright-yellow  band 
is  seen  at  solar  D.  On  account  of  the  universal  occurrence  of  sodium, 

*  Solubility  of  SrSO4  and  CaSO4  in  H2O— !SrSO4: 1000H2O;  !CaSO4:400H2O. 
t  See  §  24  and  §  31,  manganese,  1,  foot-note. 

JSalts  of  the  alkalis  are  as  a  rule  soluble.  Hence,  for  their  identification  we  rely 
upon  the  well-known  fact  that  they  impart  characteristic  colors  to  the  flame. 


QUALITATIVE    ANALYSIS  39 

it  is  only  reported  as  a  component  of  an  assay  when  the  yellow  color 
of  the  flame  or  spectrum  is  intense  and  continued  for  some  time. 

POTASSIUM  forms  the  colorless  ion  K+. 

Identification:  The  potassium  ion  colors  the  flame  violet-red, 
masked  by  sodium.  To  eliminate  the  latter,  the  flame  is  observed 
through  a  cobalt  (blue)  glass. 

AMMONIUM  forms  the  colorless  ion  NH4+. 

Identification:  1.  Ammonia  (NH3)  is  expelled  from  its  compounds 
by  the  fixed  alkalis  (§  24)  and  recognized  by  its  odor,  when  in 
quantity,  or  by  changing  the  color  of  red  litmus-paper  to  blue. 


NOTE. — Having  read  the  book  consecutively  to  this  point,  the  student  will 
now  pass  to  Chapter  VIII. 


CHAPTER  VI 


SYSTEMATIC   ANALYSIS   FOR  THE  ACIDIC  IONS 

THEIR   IDENTIFICATION 

34.  The  acids  likely  to  be  met  with  in  the  course  of  an  ordinary 
analysis  are  the  following:  Acetic,  oxalic,  tartaric,  citric;  sulphuric, 
sulphurous,   chromic,   boric;  hydroiodic,   hydrobromic,    hydrocyanic, 
hydroferro-cyanic,     hydroferri- cyanic,    hydrochloric;  nitric,    chloric, 
phosphoric,    hydrofluoric,     carbonic,    hydrosulphuric,   silicic.     These 
acids  are  divided  into  the  four  groups  named  in  §  28  by  the  tests  indi- 
cated below.     In  the  systematic  analysis  of  a  substance,  unless  it  is 
distinctively  a  metal  in  appearance  it  is  assumed  to  be  a  salt.     If  no 
acid  is  found  (and  the  substance  is  not  a  metal),  it  is  an  oxide  or  an 
hydroxide;  but  it  is  still  technically  termed  "a  salt."  The  portion  of 
ihe  salt  taken  for  analysis  is  called  the  assay. 

35.  Grouping  and  identifying  the  acid. — In  order  to  locate  the 
group  of  the  acid,  we  proceed  in  the  following  manner: — 

1st.  Heat  a  small  portion  of  the  assay — about  0.5  of  a  gram — on  a 
platinum  foil  or  a  porcelain-crucible  lid,  very  gently  at  first  and  then 
more  strongly,  and  look  for  signs  of  charring.  The  student  must  be 
careful  not  to  mistake  mere  change  of  color  (blackening)  for  char- 
ring. A  little  preliminary  practice  with  sugar  will  enable  him  to  rec- 
ognize the  change  meant.  If  charring  occurs,  notice  the  odor;  if  it  is 
like  that  of  burnt  sugar,  organic  acids  may  be  present.  Proceed  as 
directed  in  A.* 

If  the  above  tests  are  not  given,  organic  acids  are  absent.  Proceed 
to  test  for  the  next  group. 

2nd.  Powder  finely  1  g.  of  the  assay,  transfer  it  to  a  porcelain  dish, 
add  5  g.  Na2COs  crystals  and  15  cc.  distilled  water,  boil  10  minutes, 
replacing  the  water  occasionally  as  it  evaporates,  dilute  to  30  cc. ,  and 

*But  let  the  student  remember  that  oxalates  do  not  char,  while  some  organic  com- 
pounds other  than  the  acids  and  their  salts  do.  Oxalic  acid  will  be  found,  however, 
In  B,  if  present. 


QUALITATIVE    ANALYSIS  41 

then  filter  in  case  the  solution  is  not  clear  or  a  residue  remains. 
This  treatment  assures  the  transposition  of  acids  of  Group  B  into  solu- 
ble sodium  salts.  Acidify  a  small  portion  with  H2SOt.  If  a  yellow 
or  orange  precipitate  appears  (sulphides  of  As,  Sb,  Sn),  acidify  about 
half  of  the  solution,  filter,  boil  till  H2S  is  all  expelled,  and  label  it 
X.  Label  the  other  half  of  the  solution  Y. 

Put  about  3  cc.  of  Y  into  a  small  conical  flask,  add  HC1  drop- 
wise  with  constant  shaking  till  effervescence  ceases  and  the  solution  is 
slightly  acid,  boil  vigorously  to  expel  absorbed  CO,,,  and  cool  by 
allowing  a  stream  of  water  to  flow  over  the  flask.  Then  add  Ba(OH)2 
to  slight  alkalinity  and  at  once  tightly  stopper  the  flask.  If  at  the  end 
of  5  minutes  a  precipitate  appears,  acids  of  the  barium-hydroxide 
group  are  present.  Proceed  as  directed  in  B. 

If  no  precipitate  forms,  the  barium-hydroxide  acids  are  absent.  Pro- 
ceed to  test  for  the  next  group. 

3rd.  Utilize  X  and  Y  from  the  preceding  group,  if  any  remains; 
otherwise,  make  some  afresh.  Acidify  2-3  cc.  of  X  (or  of  Y  if 
there  be  no  X)  with  HNO3  and  add  a  few  drops  of  AgNOs.  If  a 
precipitate  appears,  acids  of  the  silver-nitrate  group  are  present.  Pro- 
ceed as  directed  in  C* 

If  no  precipitate  forms,  the  silver  nitrate  acids  are  absent.  Proceed 
to  test  for  the  next  group. 

4th.  Portions  of  the  *  'original  substance"  are  used  in  testing  for 
members  of  the  fourth  group.  Proceed  as  directed  in  D. 

A.— ORGANIC  GROUP 

i.  HC2H3O2. — a.  Treat  1  g.  of  the  dry  salt  in  a  test  tube  with 
about  5  cc.  of  H2SO4,  and  warm  gently.  The  presence  of  acetic  acid 
is  revealed  by  its  well-known  pungent  odor. 

b.  At  once  add  2-3  cc.  of  C2H5OH  and  a  few  drops  of  H2SO4  to 
the  contents  of  the  tube  and  heat  to  boiling.     Acetic  acid  is  revealed 
by  the  etherial,  fruity  odor  of  ethyl  acetate. 

c.  Boil  1  g.  of  the  salt  a  minute  or  so  with  5  cc.  of  H2O,  filter,  and 
add  a  drop  of  FeCl3  to  the  filtrate.     Acetic  acid  is  revealed  by  the 

*A.  white  precipitate  indicates  either  of  the  first  five;  a  colored  precipitate  indi- 
cates H.,Fe(CN)6orH2CrO4;  but  if  HaS  is  present,  all  these  precipitates  may  be  black. 


42  QUALITATIVE    ANALYSIS 

formation  of  a  deep-red  coloration.     On  boiling,  Fe(OH)3  is  precipi- 
tated by  hydrolysis. 

a.  2M+,  2C2H3O2-+2H+,  SO;-=2M+,  SO"+[2HC2H8O2]. 


b.  H+,  C2H302-+C2H+, 

c.  3H+,  3C2H302-+Fe+++,  3Cr=3H+,  301"+  [Fe(C2H2O2)3]. 

2.  H2C2O4.  —  Put  2  g.  of  the  dry  salt  in  a  porcelain  dish  with  15 
cc.  of  distilled  H2O,  add  5  g.  of  Na2CO3  crystals,  and  boil  for  10 
minutes,  replacing  the  water  as  it  evaporates.  Filter,  and  label  the 
filtrate  Z. 

Acidify  2  cc.  of  Z  with  HC2H3O2  and  add  a  few  drops  of  CaSO4. 
The  presence  of  oxalic  acid  is  revealed  by  a  white,  finely-pulverulent 
precipitate.  If  no  precipitate  appears  after  heating  and  standing  10 
minutes,  oxalic  acid  is  absent. 

2Na+,  C2O"  +  Ca++,  SO"  =2Na+,  SO4~+  [CaC12OJ. 


3.  H2C4H4O6.  —  Acidify  the  remainder  of  Z  with  HCl,  and  boil  to 
expel  absorbed  CO2.  Add  NH4OH  to  bare  alkalinity,  boil,*  then  add 
1-2  cc.  of  NH4C1,  and  then  about  2  cc.  of  CaCl2.  Shake  vigorously, 
and  allow  to  stand  10  minutes.  Tartaric  acid  is  revealed  by  a  white 
precipitate.  Confirm  the  test,  thus:  Filter,  and  reserve  the  filtrate  for 
4.  Wash  the  precipitate  with  hot  water,  transfer  it  to  a  porcelain 
dish,  add  5  cc.  NaOH,  and  stir  well  for  a  minute.  The  precipitate 
dissolves.  Dilute  to  15-20  cc.,  and  boil.  The  separation  of  a  white 
precipitate  proves  the  presence  of  tartaric  acid. 

If  no  precipitate  is  given  by  CaCl,,  tartaric  acid  is  absent. 
2M+,C4H4O6---fCa++,  2Cr=2M+,  2C1~+  [CaC4H4O6]  . 

The  presence  of  NH4C1  facilitates  the  precipitation  of  CaC4H4O6  by 
the  NH4  ions  robbing  Ca(OH)2  —  formed  by  hydrolysis  —  of  OH  ions 
to  make  relatively  undissociated  [NH4OH],  thus  keeping  the  mass 
of  Ca  ions  relatively  great.  Calcium  tartrate  dissolves  in  a  concen- 
trated solution  of  NaOH,  owing  to  the  formation  of  undissociated  solu- 

*After  expelling  excess  of  NH3  the  solution  is  neutral,  and  oxalic  acid,  if  present^ 
will  not  be  precipitated  appreciably  by  CaCl2. 


QUALITATIVE    ANALYSIS  43 

ble  complexes;  but  it  is  reprecipitated  upon  dilution  ou  account  of 
reionizatiou  in  which  Na+  and  OH~  remain  practically  in  the  ionic 
state  while  Ca++  and  C4H4O6  reunite. 

4.  H3C6H5O7.  —  Concentrate  the  filtrate  from  3  by   evaporation  to 
about  5  cc.  ,  filter  if  not  clear,  mix  with   three  times  its  volume  of 
C2H.OH,  and  add  a  few  drops  of  CaCl2.     The  formation  of  a  white 
precipitate  points  to  citric  acid.      Confirm  the  test,  thus:  Filter,  wash 
with  alcohol,  and  dissolve  on  the  filter  with  the  least  possible  amount 
of  HC1,  add  NaOH  to  alkalinity,    and  boil.     The  separation  of  a 
heavy  white  precipitate  shows  the  presence  of  citric  acid. 

If  no  precipitate  is  obtained,  citric  acid  is  absent. 
Calcium  citrate  is  quite  soluble  in  water  but  not  in  alcohol. 

B.  —  BARIUM-HYDROXIDE  GROUP 

5.  H2SO4>—  Acidify  2  cc.    of  Y  with  HC1,    and  add  a  drop  of 
BaCl2.     A  white  precipitate  indicates  sulphuric  acid. 

2Na+,  S04—-{-Ba++,  2Cl-=2Na+,  201'+  [BaSOJ  . 

6  H2SO3.  —  Acidify  2  cc.  of  Y  with  HC1  and  immediately  expose 
to  the  evolved  gas  a  drop  of  a  mixture  of  ferric  chloride  solution  and 
potassium  ferricyanide  solution  —  equal  parts  —  in  the  loop  of  a  plati- 
num wire.  A  blue  precipitate  in  the  drop  of  reagent  indicates  sul- 
phurous acid.  If  H2S  is  present  (D),  some  HC2H3O2  should  be 
introduced  into  Y  before  its  complete  acidification  with  HC1. 

Sulphurous  acid  reduces  Fe+++  to  Fe++;  and  with  ferrous  salts  a 
ferricyanide  yields  TurnbulV  's  blue: 


7.  H2CrO4.—  Acidify  2   cc.  of  Y  with  HC2H3O2  and  add  a  few 
drops  of  Pb(C2H3O2)2.     A  yellow  precipitate  shows  chromic  acid. 
2Na+,  CrO4~  +Pb++,  2C2H3O-=2Na+,  2C2H3O2-+  [PbCrOJ. 


8,  H3BO3.  —  Evaporate  2-3  cc.  of  Y  to  dryness,  cool,  moisten  with 
2  or  3  drops  of  H2SO4,  add  a  little  glycerine,  mix  with  the  glass  rod, 
and  bring  some  of  the  mixture  into  a  colorless  flame  on  a  clean  plati- 
num wire.  A  green  color  imparted  to  the  flame  shows  boric  acid. 


44  QUALITATIVE     ANALYSIS 

Sulphuric  acid  liberates  H3BO3  from  the  solution  (]Na2B4O7),  and 
this  reacts  with  glycerine  to  form  an  etherial  salt  that  is  easily  decom- 
posed by  heat.  The  boron  ion  colors  the  flame. 

9.  H3C2O4.  —  If  no'  other  acid   of  this  group  has  been  found,  the 
group  test  will  be  ascribed  to  oxalic  acid.     Proceed  as  directed  above 
(A,  2). 

C.—  SILVER-NITRATE  GROUP 

10.  HI.—  To  2  cc.  of  X  (or  of  Y  if  there  is  no  X)   add  2  cc.    of 
HNO3,    2   or  3  drops  of  K2CrO4,    and  2  cc   of  CS2,  and  shake.     A 
violet  color  in  the  CS2  shows  hydroiodic  acid. 


Na+,  I-+H+,  NO3-=Na+,  NO3--f-[HI]; 

[21]  . 


2H+,  2I-+Cr204-=  [H2CrQJ 

n.  HBr.  —  a.  HI  is  absent:  Utilizing  the  solution  used  in  10, 
pass  a  few  bubbles  of  chlorine  gas  through  it,  and  shake.  A  yellow- 
ish-red color  in  the  CS2  (Br)  shows  hydrobromic  acid. 

b.  SI  is  present:  If  HI  was  found  in  10,  it  must  be  removed  be- 
fore testing  for  HBr.  This  is  done  by  filtering  out  the  iodine-saturated 
CS2  by  passing  the  fluid  through  a  wet  filter.  The  CS2  remains  on  the 
filter.  Treat  the  filtrate  again  with  HNO3,  K2OO4,  and  CS2,  filter, 
and  repeat  till  no  further  trace  of  iodine  is  seen.  Now  treat  the  clear 
filtrate  with  chlorine  gas  and  CS2,  as  directed  in  a. 

12.  H(CN).—  Acidify  2-3  cc.  of  Y  with  H2SO4,*  and  imme- 
diately expose  to  the  evolved  gas  a  drop  of  (NH4)2S  in  the  loop  of  a 
platinum  wire  supported  in  a  cork  which  rests  loosely  on  the  mouth  of 
the  test  tube.  After  10  minutes  shake  the  drop  onto  a  porcelain  cruci- 
ble lid,  evaporate  it  to  dry  ness  at  very  gentle  heat,  and  touch  the  res- 
idue with  a  drop  of  FeCl3.  A  deep-red  coloration  shows  hydrocyanic 
acid. 

2Na+,  2CN-+2H+,  SO4~=2Na+,  SO"+  [2HCN]; 

(NH4)2Sx-fH+,CN-=NH4SCN+H2S; 
3NH4-SCN-{-Fe+++,  Cl3-=3(NH4+,Cr)-h  [Fe(CNS)J. 


^Hydrocyanic  acid  gas  is  extremely  poisonous.  Hence,  the  tube  for  its  generation 
should  be  arranged  under  the  hood  before  adding  the  H8SO4.  Cyanides  will  not  be 
given  the  student  as  an  "unknown"  for  determination.  If  a  practicing  chemist  has 
reason  to  suspect  the  presence  of  this  acid  in  a  substance,  he  assures  himself  upon 
the  point  by  making  the  above  test  before  taking  any  odor  evolved  in  the  course  of  the 
analysis. 


QUALITATIVE    ANALYSIS  45 

13.  H4Fe(CN)6,— Acidify  2  cc.  of  X  (use  Y  if  there  be  no  X) 
with  H2SO4  and  add  a  drop  of  FeCl3.     A  blue  precipitate  (Prussian 
blue)  shows  hydroferrocyauic  acid. 

3K4-Fe(CN)6+4Fe-Cl3=12(K+,  01')  +  [Pe,(Fe(CN).)J. 

If  HI  is  present  it  must  be  removed  (11,  6)  before  applying  this 
test,  as  otherwise  it  may  reduce  Fe+++  to  Fe++  and  thus  prevent  the 
reaction. 

14.  H3Fe(CN)6.— Prepare  a  little  fresh  FeSO4  by  treating  a  small 
quantity  of  iron  filings  with  H2SO4.    Add  a  drop  of  this  to  2  cc.  of  X 
(use  Y  if  there  be  no  X)  acidified  with  H2SO4.     A  dark-blue  pre- 
cipitate (TurnbulFs  blue)  shows  hydroferricyanic  acid.     A  light-blue 
color  will  be  caused  by  H4Fe(CN)6,  if  present  (13). 

2K3-Fe(CN)6+3Fe-S04==3(K2+,  SQ4")+ [Fe3(Fe(CN)6)2]. 

15.  MCI.— If  neither  HI,  HBr,  H(CN),  or  H4Fe(CN)6  has  been 
found  and  the  group-test  was  a  white  precipitate,  it  can  be  due  to  hydro- 
chloric acid  only,  and  no  further  test  for  it  need  be  made. 

If  the  group-test  gave  a  black  precipitate  (H2S),  cover  it  with 
NH4OH  in  a  porcelain  .dish,  mix  well  with  the  glass  rod,  filter,  «nd 
acidify  the  filtrate  with  HNO3.  A  white  precipitate  shows  hydro- 
chloric acid. 

If  any  acid  of  the  group  specified  above  has  been  found  in  the 
course  of  analysis,  it  must  be  removed  before  testing  for  HC1.  Pro- 
ceed thus:  Acidify  5-10  cc.  of  X  (use  Y  if  there  be  no  X)  with 
HNO3,  add  2  cc.  of  AgNO3,  shake  well,  decant  the  liquid  through  a 
filter,  wash  the  precipitate  twice  by  shaking  it  with  water,  pouring  the 
decantations  through  the  same  filter,  and  test  the  precipitate  in  the 
following  manner: — 

a.  HI  only  was  found:    Stir   the    precipitate    thoroughly   with 
NH4OH,  filter,  and  acidify  the  filtrate  with  HNO3.      A  white  pre- 
cipitate shows  hydrochloric  acid. 

b.  HBr  was  found,  and  HCNis  absent:  Cover  the  precipitate  in  a 
test  tube  with  (NH4)2CO3,  pass  CO2  through  the  mixture  for  a  short 


46  QUALITATIVE    ANALYSIS 

time,  warm  (do  Dot  boil)  the  mixture  with  occasional  shaking  for  10 
minutes,  filter,  and  acidify  the  filtrate  with  HNO3.  A  white  precip- 
itate shows  hydrochloric  acid. 

c.  HCN,  or  H^Fe(CN)6,  or  E3Fe(CN)6  was  found :  Dry  the 
precipitate  in  a  porcelain  crucible  by  gentle  heat  (below  100°),  then 
ignite  to  low  redness,  and  cool.  This  breaks  up  the  cyanogen  com- 
pounds and  expels  their  (CN)~.  Cover  the  residue  with  a  piece  of 
zinc  in  close  contact,  add  5  cc.  H2SO4,  and  allow  the  metathesis  to 
continue  an  hour.  This  reduces  AgCl  to  [Ag]  and  ZnCl2.  Pour  off 
the  liquid  and  add  to  it  a  few  drops  of  AgNO3.  A  white  precipitate 
shows  hydrochloric  acid. 

Zn++,  2Cr+2Ag+,  2NO.-=Zn++,  2NO^-f  [2AgCl]. 

D.— ORIGINAL-SUBSTANCE  GROUP 

1 6.  HNO3,— a.   H2  Cr07  was  not  found  (in  F):  To  a  little  of  the 
salt  on  a  crucible  lid,  add  a  drop  of  phenol-sulphonic  acid  and  then  a 
drop  or  so  of  NH^OH.     An  intense  yellow  color  reveals  the  presence 
of  nitric  acid. 

3HNOs+C6H5HSO4+NHpH=H2S04+3H,0+2N08+ 
[C6H5N02NH2OH]» 

b.  H2CrO^  was  found  (1 ):  Nearly  (but  not  quite)  neutralize  2 
cc.  of  Y  with  HC1,  add  Pb  (C2H3O2)2,  and  filter  off  the  PbCrO,. 
Evaporate  a  few  drops  to  dryness  on  a  crucible  lid  at  gentle  heat 
(not  above  100°),  and  test  the  residue  as  directed  in  a. 

17.  H2CO3. — Treat  1  g.  of  the  salt  in  a  test-tube  with  HC1.     If  it 
dissolves  with  effervescence,  hold  a  drop  of  Ba(OH)2  in  the  loop  of  a 
platinum  wire  in  the  escaping  gas.     The  formation  of  a  white  pre- 
cipitate (BaCO3)  shows  carbonic  acid. 

18.  HF. — Mix  about  0.5  g.   of  the  powdered  salt  with  an  equal 
bulk  of  pulverized  fused   KHSO4,  transfer  the  mixture  to  a  small 
ignition  tube  and  heat  it  strongly  for  a  minute.      With  a  file  cut  off 
the  ignition-tube  below  the  top  of  the  assay,  wash  out  the  tube  and 
dry  it  in  the  flame.     The  presence  of  an  opaque  ring  of  etched  glass 
a  little  above  where  the  assay  stood  shows  hydrofluoric  acid. 


QUALITATIVE    ANALYSIS  47 

19.  H2S, — a.    Cyanogen  acids  are  absent :  Treat  1  g.  of  the  salt  in 
a  test-tube  with  HC1.     Hydrosulphuric  acid  is  revealed  by  its  well- 
known  odor. 

b.  Cyanogen  acids  are  present  (12,  13,  14):  Hold  a  strip  of  filter 
paper  freshly  dipped  into  Pb(C2H3O2)2  in  the  gas  obtained  as  directed 
in  a.  The  blackening  of  the  paper  (PbS)  shows  H2S. 

If  the  substance  for  analysis  contains  a  soluble  sulphide,  the  pres- 
ence of  H2S  will  have  been  indicated  in  making  the  general  test  for 
Group  II.  If  the  salt  contains  certain  sulphides  insoluble  in  water 
but  capable  of  metathesis  with  a  solution  of  Na2CO3,  the  general  test 
for  Group  III  will  be  black  (see  foot-note).  If  H2S  was  found  at 
either  of  these  stages  of  the  work,  it  is  not  necessary  to  make  the 
tests  detailed  above  (a,  6). 

20.  HC1O3. — Boil  1  g.  of  the  salt  with  5  cc.  of  water,  filter,  render 
the  filtrate  faintly  blue  with  indigo,  and  acidify  with  H,SO4.    Chloric 
acid,  if  present,  reveals  itself  by  bleaching  the  solution. 

21.  H3PO4. — Dissolve  0,5  of  the  salt  in  HNO3  at  gentle  heat,  add 
an  equal  volume  of  water,  filter  if  not  clear  or  a  residue  remains,  and 
evaporate  the  filtrate  almost  to  dryness.     If  a  solid  separates  in  pro- 
cess of  evaporation,  redissolve  it  in  the  least  possible  amount  of  water. 

a.  As,    Sb,  and  Sn    are    present    (§37,11,1,2)    but  HltFe(CN')6 
is  absentC\.S)  :  Remove  these  metals  by  heating  the  solution  and  passing 
H2S  till  it  occasions  no  further  precipitation.     Filter  through  a  small 
wet  paper.     Boil  the  filtrate  till  H2S  is  all  expelled,  and  add  0.5  cc. 
of   the  solution  to  4  cc.    of  (NH4)2  MoO4.     A   yellow   precipitate 
shows  phosphoric  acid. 

b.  H,<Fe(CN)6  is  present:  If  As,  Sn,  Sb  are  present,  remove  them 
as  shown  in  a,  and  then  remove  ferrocyanic  acid  from  the  filtrate;  if 
they  are  absent,  remove  ferrocyanic  acid  from  the  solution  at  once, 
thus:    Treat  the  solution  with  ZnSO4   till  no  further   precipitation 
occurs,  filter,  and  test  the  filtrate  as  in  a. 

c.  The  metals  and  acids  named  above  are  all  absent :  At  once  treat 
the  solution  obtained  as  directed  in  the  first  paragraph  with   (NH4)2 
MoO4,  as  shown  in  a. 


48  QUALITATIVE    ANALYSIS 

22.  H2Si03.— Boil  2  g.  of  the  salt  with  20  cc.  of  water  for  5 
minutes.  Filter,  and  reject  the  filtrate.  Dry  the  residue,  pulverize 
it,  and  fuse  0.2-0.5  of  a  grain  of  it  in  a  platinum  cup  (or  on  platinum 
foil)  with  three  times  its  bulk  of  mixed  Na2CO3  and  KNO3  (3:1). 
Keep  it  in  a  state  of  fusion  for  at  least  5  minutes.  Dissolve  the  melt 
by  boiling  in  5  cc.  of  water,  filter  if  not  clear,  "and  acidify  with  HC1. 
A  flocculent  white  precipitate  indicates  silicic  acid.  Confirm  the  test 
by  evaporating  the  fluid  with  its  precipitate  to  dryness,  igniting,  cov- 
ering with  water  and  rubbing  any  insoluble  portion  in  the  bottom  of 
the  dish  with  a  glass  rod.  If  H2SiO3  (now  SiO2-)  is  present,  a  dis- 
tinct grittiness  will  be  perceived. 


CHAPTER  VII 

SYSTEMATIC  ANALYSIS  FOR  THE  BASIC  IONS 

36.  In  beginning  the  analysis  for  basic  ions,  two  cases  may  arise — i. 
The  substance  is  distinctly  metallic ;  ii.  The  substance  is  obviously 
not  metallic. 

i.  SOLUTION  OF  A  METAL.*— Treat  0.25-1.0  g.  of  the  metal  with 
HNO3  and  (unless  solution  in  the  cold  acid  begins  freely  at  once)  heat 
to  boiling: — 

a.  Solution  is  complete :  This  shows  Au,   Pt,  Sn,   and  Sb  are  cer- 
tainly absent.     Dilute  with  eight  volumes  of  water  and  proceed  as 
directed  in  I. 

b.  A  metallic  residue  will  not  dissolve:  Decant  the  nitric-acid  solu- 
tion, wash  the  residue  with  a  little  water,  and  add  the  wash-water  to 
the  decanted  acid-solution.     Evaporate  a  few  drops  of  this  solution  in 
a  porcelain  dish.     If  a  solid  residue  is  left,  proceed  with  the  solution 
as  directed  in  a. 

The  metallic  residue  is  next  dissolved  in  a  little  aqua  regia,  diluted 
with  8  volumes  of  water,  and  tested  for  gold  and  platinum  (II,  7). 

c.  The  metal  dissolves,  and  a  white  powder  is  formed  in  the  process  : 
Dilute  with  water  and  boil;  if  the  solution  clears  up,  proceed  as  directed 
in  a.     If  a  precipitate  still  remains,  Sn  or  Sb  is  present  (H2SnO3, 
HSbO3).     Filter,   wash  the  precipitate  on  the  filter  twice  with  hot 
water,   adding  the  first  wash-water  to  the  filtrate.     Label  this  solu- 
tion "I."     Now  puncture  the  bottom  of  the  filter  with  the  glass  .rod 
and  wash  the  precipitate  into  a  small  conical  flask  with  spurts  of  water 
from  the  wash-bottle,    heat  to  boiling,    and  saturate  with  H2S.     A 
colored  precipitate  (orange,   red,  yellow,  brown)  indicates  antimony 
or  tin  or  both.     To  separate  and  identify  each  when  both  occur,  treat 
this  precipitate  as  directed  in  II,  Arsenic  Sub-group,  2. 

Now  proceed  with  the  filtrate  labeled  "I"  as  directed  above  in  b. 

*Thls  includes  alloys. 


50  QUALITATIVE    ANALYSIS 

d.  The  metal  dissolves,  leaving  a  black  powder:  This  is  carbon 
(graphite).  Collected  and  heated  on  a  platinun  foil,  it  burns  without 
residue.  Iron  and  some  other  alloys  may  contain  a  considerable 
amount  of  graphite. 

ii.  SOLUTION  OF  A  SALT.  * — a.  The  substance  dissolves  in  water:  Treat 
0*5  g.  of  the  substance  in  a  test-tube  or  small  conical  flask  with  15  cc. 
of  water,  stopper  with  the  thumb  and  shake  thoroughly.  If  complete 
solution  ensues,  proceed  as  drected  in  I;  if  solution  is  not  complete 
in  cold  water,  boil  till  the  fluid  becomes  clear  and  then  proceed  as 
directed  in  I. 

b.  The  assay  does  not  dissolve  in  water  but  does  so  in  hydrochloric 
acid :     Having  tried  the  effect  of  water  as  directed  in  a,  and  a  rt^idue 
remaining,    allow  the   solid  to  subside,  decant  the  water  through  a 
filter  and  evaporate  a  few  drops  in  a  porcelain  dish  or  on  a  platinum 
foil.      In  case  a  solid  residue  is  left  from  evaporation,  label  the  water- 
solution  ''I"  for  subsequent  treatment  as  directed  in  a;  if  there  is  no 
residue    from    evaporation,    reject    the  water.     To    the    residue  not 
affected  by  water,  add  5  cc.  HC1  and  boil;  if  it  dissolves  completely, 
dilute  to  30  cc.  with  water  and  proceed  as  directed  in  II.      In  case 
HC1    does  not  effect  complete  solution,   allow  subsidence  to  occur, 
decant  the  fluid  through  a  filter,   evaporate  a  few  drops  in  order  to 
determine  if  any  solution  has  been  effected ;  if  so,  label  it  "Dil.  HCI;" 
if  not,  reject  the  filtrate.      Now  treat  the  residue  not  affected  by  HC1 
with  5   cc.    of  HCI,  and  boil.      If  it  dissolves  completely,  dilute  to 
20  cc.,  add   the  partial  solution    (if  any)    obtained  with    HCI,    and 
proceed  as  directed  in  II. 

c.  The  assay  non-soluble  in  water  and  hydrochloric  acid  dissolves  in 
nitric   acid :     A  residue    remaining  after   treatment  with  water  and 
hydrochloric    acid    as    directed  above    may    dissolve  in    nitric  acid. 
Decant  the  hydrochloric  acid  and  wash  the  assay  by  shaking  it  with 
15  cc.  of  water,  allowing  subsidence  to  occur  and  then  decanting;  the 
water.      Now  cover  the  assay  with   5  cc.  HNO3,   and  boil.      Unless 
solution  is  complete,  pour  off  a  few  drops  of  the  acid,  dilute  with  an 
equal  volume  of  water,  and  evaporate;  if  a  solid  residue  is  left,  decant 

This  word  is  here  used  technically  (see  g  34). 


QUALITATIVE    ANALYSIS  51 

all  the  nitric  acid,  dilute  to  30  cc.,  and  label  "I"  for  subsequent 
treatment  as  in  a.  If  solution  was  complete,  dilute  to  30  cc.,  and 
proceed  as  directed  in  I. 

d.  The  assay  non-soluble  in  H2  0,  HCl,  HNO^  dissolves  in  aqua 
regia :     Treat  a  residue  remaining  after  trying  the  methods  a,  b,  c, 
with  aqua  regia.     If  solution  occurs,  dilute  to  30  cc. ,  and  proceed  as 
directed  in  II. 

e.  The  assay  is  not  affected  by  water  or  acids:    Pulverize  0*5  g.  of  the 
salt  and  fuse  it  with  3  times  its  bulk  of  the  mixture  Na2CO8    (three 
parts)    KNO3  (one  part)  in  a  platinum  spoon.     Dissolve  the  melt  by 
boiling  in  15  cc.  of  water,  and  filter  if  not  clear.     Acidify  with  HNO3. 
If  no  precipitate  occurs,  proceed  as  directed  in  I,     If  a  precipitate 
occurs  (H2SiO3,  S,  H2SnO3,  HSbO3),  filter,  and  label  the  filtrate  "I" 
for  subsequent  treatment  as  directed  in  a.     If  the  precipitate  is  light- 
yellow  and  burns  with  a  pale-blue  flame  with  evolution  of  SO2,  it 
points  to  the  salt's  being  a  poly-sulphide;  if  silicic  acid  has  previously 
been  found  (§35,  22),   it  will  appear  here  also.     But  if  the  precip- 
itate is  voluminous  and  white,  tin  and  antimony  are  probably  present. 
Test  it  as  shown  in  i,  c. 

37.  Grouping  and  identifying  the  basic  ions. — 
I. —HYDROCHLORIC- ACID  GROUP 

To  the  solution"!"  (obtained  by  methods  detailed  in  section  36),  add 
1-2  cc.  HCl,  stopper  and  shake  thoroughly.  If  no  precipitate  appears, 
members  of  this  group  (Ag,  Pb,  THg)  are  absent;  proceed  to  Group 
II.  If  a  precipitate  is  formed,  filter,  and  test  the  filtrate  with  a  drop 
of  HCl.  If  it  remains  clear,  precipitation  is  complete;  otherwise  you 
must  repeat  the  process  of  precipitation  and  filtration  until  no  cloud- 
iness is  observed  upon  addition  of  a  drop  of  the  reagent.  Label  the 
filtrate  "II". 

1.  LEAD:    Wash  the  precipitate  on  the  filter  with  5  cc.  of  boiling 
water,  and  add  K2CrO4  to  the  filtrate.     A  yellow  precipitate  shows 
lead  (§29).     However,  lead  may  be  present  and  not  show  itself  here. 

2.  OUS-MERCURY:    Pour  about  3  cc.  of  NH/)H  over  any  residue 


52  QUALITATIVE    ANALYSIS 

left  on  the  filter.     If  it  turns  black,   monovalent  mercury  is  shown 
(§29). 

3.  SILVER:  Acidify  the  ammonia  filtrate  with  HC1.  A  white  pre- 
cipitate shows  silver  (§29). 

II.  —SULPHURETTED-HYDROGEN  GROUP 

The  filtrate  from  I  (or  solution  "II",  §36)  must  be  moderately 
concentrated  (§26,  second  paragraph),  only  slightly  acid,  and  free 
from  an  oxidizing  acid  (HNO3);  because,  first,  H2S  being  a  weak 
acid  its  dissociation  is  repressed  by  a  stronger  acid  (§24),  and,  second, 
an  oxidizing  acid  decomposes  it : 

2H+,  S  +  H+,  NO;=[H20]  +  (H+,  NO,)  +  [8]. 

In  case  HNO3  is  present,  evaporate  to  complete  dryness,  ignite  the 
residue  (nitrates  are  decomposed  at  red  heat),  cool,  dissolve  the  resi- 
due in  a  little  HC1,  dilute  to  30-40  cc.,*  pour  5  cc.  into  a  test-tube 
labeled  "Au-Pt,"  and  reserve.  Saturate  the  remainder  with  H2S.f 
If  no  precipitate  appears,  members  of  this  group  (Pb,  "Hg,  Cu,  Cd, 
Bi,  As,  Sn,  Sb,  Au,  Pt)  are  absent;  proceed  to  Group  III.  If  a 
precipitate  is  formed,  filter,  and  test  the  filtrate  with  H2S  till  precipi- 
ation  is  complete.  Wash  the  precipitate  with  hot  water,  add  the 
first  washing  to  the  filtrate  and  label  it  "III." 

If  HNO3  is  not  present,  reserve  5  cc.  for  gold  and  platinum,  and 
saturate  the  rest  with  H2S,  etc.,  as  above. 

SEPARATION    OF   THE    ARSENIC    SUB-GROUP. 

Loosen  the  edges  of  the  filter,  lift  it  from  the  funnel  to  the  palm  of 
one  hand  and  unfold  it  so  that  it  lies  flat,  cover  it  with  a  small  porce- 
lain dish,  and  by  a  quick  reversal  of  the  hands  bring  the  filter  upper- 
most. Beginning  at  the  edge  of  the  filter,  press  the  paper  gently,  so 
as  to  cause  the  precipitate  to  adhere  to  the  dish,  and  then  taking  the 
paper  by  one  edge  strip  it  from  the  precipitate.  If  some  precipitate 
remains  with  the  paper,  wash  it  into  the  dish  with  a  few  sharp  spurts 

*  The  appearance  of  a  white  precipitate  (BiOCl,  Sb4  O5C12)  due  to  hydrolysis  will 
not  interfere  with  metathesis  by  H2S. 

t  This  may  require  a  slow  stream  of  gag  for  15  minutes  with  constant  shaking  of 
the  flask.  The  metathesis  is  more  rapid  if  the  solution  is  kept  warm. 


QUALITATIVE    ANALYSIS  53 

of  water,  using  as  little  as  possible.  Pour  over  the  precipitate  barely 
enough  (NH4)2Sx  to  cover  it  well  and  warm  (not  boil)  on  asbestos 
with  constant  stirring  for  five  minutes.  Filter,  wash  the  precipitate 
on  the  filter  with  hot  water  to  which  some  of  (NH4)0Sx  has  been 
added,  and  unite  the  wash-water  with  the  filtrate  (§  30).  Label 
the  filter  paper  with  its  precipitate,  "Copper  Sub-group,"  and  reserve. 
Acidify  the  filtrate  with  HC1.  If  no  precipitate  appears,*  As,  Sh, 
and  Sb  are  absent.  Proceed  with  the  copper  sub-group.  But  if  a 
colored  (lemon-yellow,  orange-red,  yellow  to  brown)  flocculent  pre- 
cipitate separates,  filter,  wash  with  hot  water,  reject  the  filtrate  and' 
test  the  precipitate  thus: — 

1.  ARSENIC:  Puncture  the  filter,  and  wash  the  precipitate  into  a 
small  conical  flask  with  sharp  spurts  of  water,  using  the  least  possible 
quantity.     Add  NH4OH  to  alkalinity,  then  10-15  cc.  (NH4)2  CO3, 
stopper  and  shake   for  five  minutes.     Filter,   wash   the   precipitate 
thoroughly,  joining  the  first  washing  to  the  filtrate.     Label  the  filter 
with  its  precipitate  "Sb-Sn,"  and  reserve.     Acidify  the  filtrate  with 
HC1.     The   formation  of  a  flocculent  lemon-yellow  precipitate  shows 
arsenic  (§  30). 

2.  ANTIMONY-TIN:    The  precipitate  labeled  "Sb-Sn"  is  now  dis- 
solved  in  the  least  possible  amount  of  aqua  regia,  diluted  with  an 
equal  volume  of  water,  filtered  from  free  sulphur,  and  tested  as  de- 
tailed in  §  30,  tin,  3  (or  4). 

SEPARATION    OF    THE   COPPER   SUB-GROUP. 

The  filter  labeled  "copper  sub-group"  is  transferred  to  a  small 
porcelain  dish  and  boiled  with  HN03 .  If  a  residue  (yellowish- 
white  or,  more  commonly  black),  remains,  add  an  equal  volume  of 
water,  and  filter.  Wash  the  precipitate  and  label  it  "nHg."  The 
filtrate  (which  should  not  exceed  a  volume  of  5-10  cc.)  is  now  diluted 
with  an  equal  volume  of  alcohol,  1  or  2  cc.  of  H2SO4  added,  heated 
to  boiling,  then  allowed  to  cool.  The  separation  of  a  white  precipitate 
(PbSO4)  shows  lead  (§  29).  Filter. 

*  The  separation  of  Sulphur' from  the  reagent  will  of  course  occur.  If  may  be 
white  and  very  fine,  or  flocculent  and  light  yellow  (sulphur-yellow)  to  brownish, 
easily  passing  through  a  filter  when  fine  and  remaining  suspended  in  the  fluid. 
Be  careful  not  to  confuse  this  separated  sulphur  with  sulphides  of  arsenic,  antimony 
and  tin. 


54  QUALITATIVE    ANALYSIS 

3.  COPPER:  Render  the  filtrate  from  PbSO4  (or  the  solution  tested 
for  lead  if  there  was  no  precipitate)  alkaline  with  NH^OH.     The  for- 
mation of  a  deep-blue  color  shows  copper  (§  30).     If  a  white  precipi- 
tate i-  formed  at  the  same  time,  this  shows — 

4.  BISMUTH:  Filter,   wash  the  precipitate,   and  confirm  as  shown 
in  §  30,  Bismuth  3,  4. 

5.  CADMIUM:  The  filtrate  from  bismuth  (or  the  ammoniacal  fluid 
if  there  was  no  precipitate)   is  to  be  tested  for  cadmium.     If  copper 
was  absent,  the  fluid  is  clear;  saturate  it  with  H^S  (§  30,  Cadmium, 
2) .     If  the  solution  is  blue,  discharge  the  blue  color  (§  30,  Copper, 
2),  and  then  saturate  with  H^S.     The  formation  of  a  yellow  precipi- 
tate shows  cadmium. 

6.  IC-MERGURY:  The  precipitate  labeled   "nHg"  is   dissolved  in 
the  least  possible  amount  of  aqua  regia,  diluted  with  an  equal  volume 
of  water,  filtered  if  not  clear,   and  treated  with  SnCl2.     A  white  to 
gray  or  black  precipitate  shows  bivalent  mercury  (§  30). 

7.  PLATINUM-GOLD  :  The  contents  of  the  test  tube  labeled  * '  Au-Pt' ' 
is  now  to  be  tested  for  the  elements  indicated.     Filter,  if  not  clear, 
and  divide  into  two  portions.     Test  the  first  for  gold  as  directed   in 
§  30,   Gold  2;  and  the  second,  for  platinum  as  shown  in  §  30,  Plati- 
num, 2. 

Ill,— AMMONIUM-SULPHIDE  GROUP 

If  oxalic  acid  was  found  (§  35  B,  9),  it  must  be  removed  from  the 
solution  labeled  "III*'  in  order  to  prevent  reduction  of  salts  of  the 
group  to  the  ous-eoadition.  Hence  two  courses  of  procedure  arise:— 

a.  Oxalic  add  is  absent:  Proceed  as  directed  in  A. 

b.  Oxalic  acid  is  present:  Add  1-2  cc.    of    HNOs  to  the  solution, 
evaporate  (under  the  hood)  to  complete  dryness,  moisten  the  residue 
with    HNO3  ,   evaporate,  and  ignite  for  a  short  time  at  low    redness. 
Dissolve  the  residue  in  HCI-   dilute  with  a  little  water,  filter  if  not 
clear,  add  2  cc.  of  NH4C1  .(§  33,  Magnesium,  1),  dilute  to  30  cc  , 
and  proceed  as  directed  in  A. 

A.  Render  the  solution  (brought  on  from  a  or  b)  alkaline  with 
NH4OH,  boil  to  expel  excess  of  ammonia  (shown  by  the  odor),  and 


QUALITATIVE    ANALYSIS  55 

add  freshly- prepared  (NHJ2S  being  careful  to  avoid  much  excess. 
If  no  precipitate  appears,  members  of  this  group  (Fe,  Al,  Or,  Co, 
Ni,  Mn,  Zn)  are  absent;  proceed  to  Group  IV.  If  a  precipitate  is 
formed,  filter  rapidly  and  wash  immediately  with  hot  water,  to  which 
a  few  drops  of  (NH4)2  S  have  been  added,  keeping  the  surface  of  the 
precipitate  covered  with  the  wash- water.  Label  the  filtrate  "IV." 

SEPARATION    OF    THE   COBALT-NICKEL   SUB-GROUP. 

At  once  treat  the  precipitate  on  the  filter  with  special  HC1  (IHC1: 
10H2O).  If  all  dissolves,  cobalt  and  nickel  are  absent.  If  a  resi- 
due remains,  It  should  be  black.  Label  the  HC1  solution  "Iron" 
and  reserve.  Examine  the  black  precipitate  insoluble  in  HC1  for — 

1.  COBALT:  Test  as  directed  in  §  31,  Cobalt,  2*. 

2.  NICKEL:     If  the  filtrate  from  the  precipitate  by    (NH4)2S  is 
brownish  colored,  nickel  is  present  (§  31,  Nickel,   1).     If  cobalt  was 
not  found  as  directed  above,  the  borax-bead  there  obtained  is  violet 
to  red-brown  in  color  (§  31,  Nickel,  2).     If  cobalt  was  found,  it  must 
be  removed  before  testing  the  precipitate  for  nickel.      Proceed   as 
directed  in  §  31,  Nickel,  3. 

SEPARATION    OF    THE    IRON   SUB-GROUP. 

The  filtrate  labeled  "Iron"  is  now  examined  for  the  remaining 
members  of  the  group.  Boil  till  the  odor  of  H2S  can  no  longer  be 
detected,  then  add  a  few  drops  of  HNOs  and  boil  again  for  2 
minutes. 

3.  IRON:  Pour  about  1  cc.  of  the  solution  into  a  test-tube,  dilute 
to  10  cc. ,  and  add  a  drop  or  two  of  KCNS.     A  red  coloration  shows 
iron  (§  31,  Iron,  3);  if  the  color  is  slightly  red,  report — "A  trace  of 
iron." 

Since  in  neutral  or  alkaline  solution  iron,  aluminum  and  zinc  form 
nearly  insoluble  precipitates  with  PO~  ,  if  phosphoric  acid  is  present 
(§  35,  D,  "21)  it  must  now  be  removed.  Hence,  two  courses  of  pro- 
cedure arise: — 

*For  the  details  of  blow-pipe  technique  see  Martin's  Qualitative  Analysis  with 
the  Blow-pipe. 


50  QUALITATIVE    ANALYSIS 

a.   Phosphoric  acid  is  absent:  Proceed  as  directed  below  in  A. 
^  b.   Phosphoric  acid  is  present :  Add  FeCl3  to  the  solution  till  a  drop 
removed  with  the  glass  rod  and  tested  on  a  porcelain  crucible  lid  gives 
a  brown  precipitate  (Fe(OH)s)  with  a  drop  of  NH4OH,  filter  from 
FeP04*,  and  proceed  with  the  filtrate  as  directed  in  A. 

A.— SEPARATION  OF  THE  ALUMINUM  SUB-GROUP 

Concentrate  the  solution  by  evaporation  to  3-5  cc.,  barely  neutral- 
ize with  (NH4)2CO3,  redissolve  any  precipitate  thus  formed  with  the 
least  possible  amount  of  special  HC1  (1HC1:  20H2O),  transfer  to  a 
small  conical  flash,  add  10  cc.  of  BaCO3-emulsionj  and  allow  meta- 
thesis to  go  on  for  several  hours,  giving  the  contents  of  the  flask  a 
thorough  shaking,  meanwhile,  about  every  15  minutes.  This  treat- 
ment precipitates  iron,  aluminum  and  chromium  as  ic-hydroxides, 
while  the  chlorides  of  manganese  and  zinc  are  not  affected  by  BaCO3 
(=Ba(OH)2  from  hydrolysis). 

•Filter  and  wash  with  hot  water,  adding  the  first  wash-water  to  the 
filtrate.  Label  the  filtrate  "Mn-Zn"  and  reserve.  The  precipitate 
is  now  to  be  tested  for  Al  and  Cr. 

a.  Not  more  than  a  trace  of  iron  and  no  H3PO^  were  found:  Dis- 
solve a  small  portion  of  the  precipitate  in  HC1,  add  H2SO4  till  it  oc- 
casions no  more  precipitation  (BaSO4),  heat  to  boiling  and  filter, 
rejecting  the  precipitate. 

4.  ALUMINUM:  Treat  2  cc.  of  the  filtrate  with  NH4OH,  adding  it 
from  a  dropping  tube  held  so  as  to  deliver  the  reagent  gently  at  the 
surface  of  the  fluid-assay.  A  gelatinous  white  precipitate  forming  a 
zone  at  the  junction  of  the  two  liquids  shows  aluminum  (§  31,  Alumi- 
num, 1,  2). 

Test  the  remainder  of  the  filtrate  for  chromium  as  directed  in  b-, 
o  II. 

b.  Iron  or  H%POk  was  found:  Fuse  a  portion  of  the  BaCO3-pre- 
cipitate  with  fusing  mixture  (1NA2CO3: 1KNO3)  in  the  platinum 
cup.  Extract  the  melt  with  a  little  hot  water,  filter  if  not  clear,  and 
acidify  the  filtrate  with  HC2H3O2. 

*The  phosphates  of  this  group,  except  that  ol  Iron,  are  soluble  in  dilute  HC1. 
Hence,  all  the  H3PO4  is  precipitated  as  the  iron  salt. 


QUALITATIVE    ANALYSIS  57 

5.  CHROMIUM:  Test  this  filtrate  for  chromium  as  directed  in  §  31, 
Chromium,  3. 

Chromium  II:  The  filtrate  brought  on  from  4  may  be  tested  for 
Cr  by  treating  it  with  NH4OH  to  excess,  filtering  off  the  precipitated 
A1(OH)3,  and  then  adding  Pb  (C2H3O2)2. 

Aluminum  II:  Transfer  another  portion  of  the  BaCO3-precipitate 
to  a  small  porcelain  dish,  add  1  g.  Na2CO3  crystals  and  0.5  Ba(OH)2 
crystals,  cover  with  10  cc.  of  water  and  boil  5  minutes  replacing  the 
water  as  it  evaporates.  Filter,  reject  the  precipitate,  acidify  the 
filtrate  with  HC1,  arid  add  NH4OH  to  the  solution  as  directed  above 
in  4. 

The  filtrate  labeled  "Mn-Zu"  is  now  to  be  examined  for  the 
elements  indicated. 

6.  ZINC:     Acidify  2  cc.  of  this  solution  with  HC 2 H3O 2 ,   and  pass 
H2S.     A  white  precipitate  shows  zinc  (§  31,  Zinc,  1). 

'  7.  MANGANJESE:  Evaporate  the  remainder  of  the  filtrate  nearly  to 
dryness  (1  cc.  or  less)  and  test  as  directed  in  §  31,  Manganese  2,  or  3. 

IV.— AMMONIUM-.CARBONATE  GROUP 

If 'the  filtrate  from  (NH4)2S  labeled  "IV"  is  colored  brown  (from 
nickel),  acidify  it  with  HC2H302  and  boil  for  5  minutes.  Filter, 
and  concentrate  to  20-30  cc.  in  case  the  volume  is  greater  than  that. 
Cool,  render  alkaline  with  NH4OH,  boil,  and  add  to  the  solution 
while  boiling  (NH4)2CO3  in  excess.  If  no  precipitate  forms,  mem- 
bers of  this  group  (Ba,  Sr,  Ca')  are  absent;  proceed  to  Group  V.  If 
a  precipitate  forms,  filter,  wash  thoroughly,  add  the  first  wash -water 
to  the  filtrate  and  label  it  "V".  •  Examine  the  precipitate  for  barium, 
strontium  and  calcium. 

..  Dissolve  the  precipitate  on  the  filter  in  the  least  possible  amount  of 
acetic  acid*,  pouring  the  same  portion  (after  passing  through  the 
filter)  over  the  precipitate  so  long  as  it  will  effect  any  solution  which 

*Barium  chromate.is,  slightly  soluble  in  HC2H3O2.  Since  Ba  is  removed  from  the 
solution  of  Ca  and  Sr  as  the  chrornate,  an  excess  of  HC2H3O2  is  to  be  avoided;  other- 
wise, the  separation  of  barium  will  not  be  complete  and  confusion  will  ensue  when 
Sr  and  Ca  are  tested  for.  : 


58  QUALITATIVE    ANALYSIS 

may  be  known  by  the  occureuce  of  effervescence  while  the  liquid  is  in 
contact  with  the  solid.  The  precipitate  should  be  wholly  soluble  in 
acetic  acid. 

1.  BARIUM:    Treat  2  cc.  of  the  acetic  acid  solution  with  K0CrO4.  If 
no  precipitate  is  formed,  barium  is  absent.      Examine  the  rest  of  the 
solution  for  strontium  and  calcium.     If  a  yellow  precipitate  is  formed, 
barium  is  present.     Add  the  reagent  to  the  rest  of  the  solution,  shake, 
and  filter.     Label  the  filtrate  "Sr-Ca"  and  reserve.     Wash  the  pre- 
cipitate, and  test  it  further  as  directed  in  §  32,  Barium,  2. 

2.  STRONTIUM:     The  filtrate  labeled    "Sr-Ca"    (or   the   solution 
brought  on,  if  Ba  was  absent)  is  now  examined  for  the  elements  indi- 
cated.    Make  it  alkaline  with  NH4OH,  then  add   (NH4)2CO3.     If 
no  precipitate  appears,  strontium  and  calcium  are  both  absent.      If  a 
precipitate  is  formed,  filter,  reject  the  filtrate,  and  wash  the  precipi- 
tate till  it  is  entirely  white  in  color.     Dissolve  it  on  the  filter  in  the 
least  possible  amount  of  HC1,  and  divide  into  two  portions     To  the 
first,  add  CaSO4,  heat  to  boiling,  and  let  stand  10  minutes.     If  no 
precipitate  appears,  strontium  is  absent,     If  a   white    precipitate   is 
formed,  strontium  is  indicated  (  §  32,  Strontium,  2. ) 

3.  CALCIUM:     If  strontium  wa<  not  found,  test  the  second  portion 
of  the  HCi-solution  at  once  for  calcium  as  indicated  below  after  the  re- 
moval of  strontium.     But  if  strontium  is  present,  it  must  now  be  re- 
moved.    Add  K2SO4  in  excess,  boil,  filter,  rejecting  the  precipitate, 
render  the  filtrate  alkaline  with  NH4OH,  and  then  add  (NH4)2  C2O4. 
A  white  precipitate  shows  calcium  (§  32,   Calcium,  1). 

V.— ALKALI-METAL   GROUP 

Concentrate  the  filtrate  labeled  "V"  to  about  15  cc.,  put  5  cc.  into 
a  test-tube  labeled  "Mg,"  and  evaporate  the  remainder  to  dryness  in 
the  hood  and  ignite  the  residue  so  long  as  white  fumes  (NH4C1)  are 
given  off.  Dissolve  the  residue  in  5  cc.  of  water,  and  filter  if  not 
clear,  and  test  this  solution  for  Li,  Na,  K. 

1.  MAGNESIUM:  Test  the  content*  of  the  test  tube  marked  4fMg" 
for  that  element  by  the  method  indicated  in  §  33,  Magnesium,  1. 


QUALITATIVE    ANALYSIS  59 

2.  LITHIUM-SODIUM-POTASSIUM:    Test  the  water  solution  reserved 
above  for  the  elements  indicated  as  directed  in  §  33. 

3.  AMMONIUM:     Transfer  the  remainder  water  solution  used  for 
the  alkali  metals  to  a  beaker,  treat  with  5  cc.  of  NaOH,  cover  with  a 
glass  plate  to  the  lower  side  of  which  you  have  caused  a  strip  of  wet 
red  litmus  paper  to  adhere,  and  warm  gently   (do  not  allow  the  fluid 
to  boil).     Ammonia  is  shown  by  the  color  of  the  paper  changing  to 
blue  (§  33,  Ammonium,  1). 


CHAPTER  VIII 

PRACTICAL  EXERCISES 

Having  mastered  the  first  five  chapters,  the  student  is  now  pre- 
pared to  proceed  with  a  consecutive  analysis  by  the  directions  set 
forth  in  Chapters  VI- VII. 

Two  CASES  ARISE:  (i)  The  substance  is  a  metal. — Begin  at  §  36,  i, 
and  proceed  consecutively.  If  but  one  basic  ion  is  found,  the  metal 
is  elemental ;  if  two  or  more  are  found,  it  is  an  alloy . 

(ii).  The  substance  is  not  metallic. — Begin  at  §34  and  determine 
the  acid,  proceeding  systematically  as  there  directed.  In  case  of  a 
mixture,  more  than  one  acid  may  be  present.  If  no  acid  is  found, 
the  "salt"  is  an  oxide  (or  hydroxide).  Then  pass  to  §36,  ii,  and, 
proceeding  consecutively,  determine  the  basic  ion  (or  ions).  If  but 
one  basic  ion  and  one  acid  ion  are  found,  the  salt  is  readily  named; 
e.  g.,  K+  and  Cl~,  =KC1.  If  several  basic  ions  are  found  with  one 
acidic  ion,  all  are  salts  of  the  same  acid;  e.  g.,  Ca++,  Cu++,  Ag+,  and 
NO3-,:=Ca(NO3(2,  Cu(NO3)2,  AgNO3.  But  if  several  acidic  ions  and 
several  basic  ions  are  found,  it  is  impossible  to  know  their  collocation 
in  the  mixture.  Simply  report — The  following  metals  **,  **,  **, 
and  acids  **,  **,  were  found;  e.  g.,  Cd++,  Ni++,  Zn++;  SO;-, 
Cl~,  being  determined  :  "The  following  metals — Cd,  Ni,  Zn,  and  the 
acids— H2SO4  and  HC1  were  found." 

Omit  no  step  of  the  systematic  scheme.  Write  out  all  reactions  in- 
volved. Endeavor  to  follow  with  "the  mental  eye"  the  changes 
from  first  to  last.  And,  as  soon  as  possible,  fix  the  whole  scheme 
of  analysis  in  your  memory.  This  will  save  you  much  time  and  add 
greatly  to  your  enjoyment  of  the  work. 

EXERCISES 

1.  Weigh  AgNO,  0-2g.,  HgNO30-2g.,  Cu(NO3)20-2g.,  Co(NO3)2 
0*2  g. ,  and  Ba(NO3)2  0'2 g.  Mix  by  pouring  from  one  paper  to 
another,  pulverize,  and  again  mix  thoroughly.  Analyze. 


QUALITATIVE    ANALYSIS  61 

2.  Weigh  CdCl2  0  -2  g. ,  FeCl3  0  -2  g. ,  MnCl2  0  -2  g. ,  CaCJ2  0  -2  g. ,  and 
NH4C1  0-2  g.     Mix,  pulverize,  remix,  analyze. 

3.  Weigh  Pb(NO3),  0'2  gv  Ni(NO3)2  0'2g,  Sr(NO3)20'2  g.,   and 
KNO3  0-2  g.     Prepare  as  shown  above,  and  analyze. 

4.  Weigh  HgCl2  0-2g.,  MgCl2  0'4g.,  NaCl  0-2g,  and  LiCl  0-lg. 
(since  this  salt  is  very  hygroscopic,    it  may  be  in  the  laboratory  in 
water-solution  only;  if  so,  pulverize  the  other  salts  and  then  add  2  or 
3  drops  of  the  saturated  solution  of  LiCl  to  the  mixture).     Prepare 
and  analyze. 

5.  Weigh  CdSO4  0-2g.,    A12(SO4)S  0-4  g.,  K2CrO4  0-2g.,   and 
ZnSO4  0  '2  g.     Prepare  and  analyze. 

6.  Weigh  FeSO4  0-2g.,  MnSO4  0-3  g.,  ZnSO4  0'3g.,  and  (NH4)2 
SO4  0  *2  g.    Prepare  and  analyze. 

7.  Weigh  AgNO,  0-lg.,  Cu(NO3)2  0-2 g.,   Bi(NOs),(if  this  salt 
is  not  available,  dissolve  O'l  g  of  the  metal  in  hot  concentrated  nitric 
acid,  and  add  this  solution  to  the  solution  of  the  other  salts.     Head 
§31,  BISMUTH).   Prepare  and  analyze. 

8.  Weigh  As2O3  0'2g.,  and  SnCl2  0'3g.     Prepare  and  analyze. 

9.  Weigh  As2O3  0'2g.,  pulverize  it,  dissolve  it  in  2-3  cc.  of  hot 
concentrated   HC1,  add  0'3  g.  of  KSbC4H2O6  dissolved  in  5  cc.    of 
hot  water,  and  analyze  for  basic  ions  only. 

10.  Weigh  As2O3   0-2 g.,    KSbC4H2O6 0 -3 g. ,  SnCl2   0-3 g.,    and 
analyze  for  basic  ions  only. 

11.  Weigh  KO2H3O2  0-5  g.,  KSbC4H2O6  0'5  g.     Full  analysis. 

12.  Weigh  H2C2O4  0'5   g.,   H2C4H4O6   0-5  g.,  and   analyze  for 
acidic  ions  only. 

13.  Make  a  full  analysis  of  2  g.  of  Na2SO3. 

14.  Make  a  full  analysis  of  1  g.  of  Na2B4O7. 

15.  Make  a  full  analysis  of  1  g.  of  KI. 

16.  Weigh  KI  0'5  g.  and  KBr  0'5  g.     Complete  analysis. 

17.  Make  a  complete  analysis  of  1  g.  of  K4Fe(CN)6. 

18.  Make  a  complete  analysis  of  1  g.  of  KsFe(CN)6. 

19.  Make  a  complete  analysis  of  NaCl  1  g.,  KI  05  g.,  and  K3Fe- 
(CN).  1  g. 


62  QUALITATIVE    ANALYSIS 

20.  Make  a  complete  analysis  of  1  g.  of  CaS. 

21.  Weigh  MgCO3,  BaCO3,  and  CaCO3,  0-5  g.  of  each.    Complete 
analysis. 

22.  Make  a  complete  analysis  of  1  g.  of  Na2HPO4. 

23.  Make  a  complete  analysis  of  1  g.  of  KC1O3. 

24.  Analyze  0*5  CaF2  for  HF  only. 

25.  Analyze  1  g.  of  sand  for  H%Si03  only. 

26.  Weigh  PbO2   0-2  g.,   CuO  0-2  g.,  MnO20'2g.,  BaOO'lg. 
Make  a  complete  analysis  (both  basic  and  acidic). 

27.  Analyze  a  five-cent  coin. 


NOTE  TO  THE  TEACHER  :  The  pupil  may  now  be  exercised  on  unknowns  un- 
til his  knowledge  and  skill  are  proved. 

Since  the  most  common  source  of  discouragement  to  the  pupil  is  found  in 
impurities,  all  reagents  should  be  made  of  pure  chemicals;  and  only  pure  chemi- 
cals should  be  given  out  for  analysis  in  the  foregoing  exercises.  No  "unknown" 
should  be  unknown  to  the  instructor. 


APPENDIX 


REAGENTS 
ACID — 

ACETIC  :  glacial  acetic  (sp.  gr.  1'04)  1  part,*  water  2  parts. 
AQUA  BEGIA  :  cone.  HC1  3  parts,  cone.  HNO3  1  part. 

HYDROCHLORIC,  CONCENTRATED  :    Sp.  gr.    1*12. 

HYDROCHLORIC,  CONG.  ORDINARY:  HC1  sp.  gr.  1/12  with  equal  volume 

of  water. 

HYDROCHLORIC,  DILUTE:  cone.  HC1  1  part,  water  4  parts. 
HYDROSULPHURIC  (H2S):  generate  the  gas  as  needed. 
NITRIC,  CONCENTRATED:  sp.  gr.  1*42. 
NITRIC,  DILUTE:  cone.  HNO3  1  part,  water  4  parts. 
OXALIC:  1  part,  water  10  parts. 
PHENOL  SULPHONIC:  phenol  1  part,  cone.  H2SO4  4  parts,  mix  in  a  small 

flask,  warming  it  occasionally  in  hot  water  till  solution  occurs. 
SULPHURIC,  CONCENTRATED:  sp.  gr.  1*84. 
SULPHURIC,  DILUTE:  cone.  FT2SO4  1  part,  water  4  parts. 
TARTARIC:  1  part,  water  10  parts. 
ALCOHOL:  C2H5OH  or  CH3OH,  90  per  cent. 

AMMONIUM — 

ACETATE:  glacial  acetic  acid  1£  parts,  NH4OH  (sp.  gr.  0'9)  1  part. 

CARBONATE:  water  10  parts,  NH4OH  (sp.  gr.  0'9)  1  part,  (NH4)2- 
CO3,  2£  parts. 

CHLORIDE:  water  10  parts,  NH4C1  1  part. 

HYDROXIDE  (sp.  gr.  0'9):    water  2  parts,  NH4OH  (sp.  gr.  0'96)  1  part. 

MOLYBDATE:  dissolve  25  g.  MoO3  in  100  cc.  water  to  which  20  cc.  NH4- 
OH  (sp.  gr.  0'90)  has  been  added,  filter,  and  add  with  con- 
stant stirring  to  250  cc.  dil.  HNO3  (cone.  HNO3  1  part, 
water  2  parts),  stand  48  hours  in  a  warm  place,  then  decant. 

OXALATE:  (NH4)2C2O42H2O  1  part,  water  40  parts. 

SULPHIDE  ((NH4)2S):  make  as  needed — saturate  30  cc.  NH4OH  (sp.  gr. 
0-90)  with  H2S,  add  20  cc.  NH4OH  (sp.  gr.  0'90)  and  40  cc. 
water. 

SULPHIDE,  POLY  ((NH4)2Sx):  saturate  the  colorless  (NH4)2S  with 
flowers  of  sulphur. 

*Part  means  grams  In  case  of  solids,  and  cubic  centimeters  for  liquids. 


64  QUALITATIVE    ANALYSIS 

BARIUM — 

CARBONATE:  suspend  BaCO3  1  part,  in  water  4  parts;  use  as  emulsion. 

CHLORIDE:  BaCl2  1  part,  water  10  parts. 

HYDROXIDE:  saturate  solution;  decant  and  use  the  clear  portion. 
CALCIUM- 
CHLORIDE  :  CaCl26H2O  1  part,  water  10  parts. 

HYDROXIDE:  see  barium. 

SULPHATE  :  saturated  solution. 
CARBON  BISULPHIDE:   CS2- 
FERRIC  CHLORIDE:   FeCl3 1  part,  water  10  parts. 

INDIGO :      add  1  part  of  pulverized  indigo  to  5  parts  of  fuming  H2SO4  with  con- 
stant stirring,  keeping  the  acid  cool  with  cold  water  around 
the  tube  or  beaker,  cover,  let  stand  48  hours,  then  pour  into 
20  times  its  volume  of  water,  mix  and  filter. 
IRON:  filings. 

LEAD  ACETATE:  Pb(C7H3O2)23H2O  1  part,  water  10  parts. 
LlME  WATER:  see  calcium  hydroxide. 
MERCURIC  CHLORIDE:  HgCl2 1  part,  water  20  parts. 
POTASSIUM- 
ACETATE:  saturated  solution. 

CHROMATE  :  K2CrO4  1  part,  water  10  parts. 

CYANIDE:  KCN  1  part,  water  20  parts. 

FERRICYANIDE:  K3Fe(CN)6  1  part,  water  100  parts. 

FERROCYANIDE  :  K4Fe(  CN  )63H2O  1  part,  water  20  parts. 

IODIDE  :  KI  1  part,  water  20  parts. 

SULPHATE:  K2SO4  1  part,  water  10  parts. 

SULPHOCYANATE:  KCNS  1  part,  water  20  parts. 
SILVER  NITRATE:  AgNO3 1  part,  water  20  parts. 
SODIUM — 

CARBONATE:  Na2CO3  1  part,  water  7  parts. 

HYDROXIDE:  NaOH  1  part,  water  10  parts. 

PHOSPHATE:  Na2HPO412H2O  1  part,  water  10  parts. 

STANNOUS  CHLORIDE:  dissolve  50  g.  of  t>n  in  hot  cone.  HC1,  add  4  times  its 
volume  of  water,  filter  if  not  clear,  and  keep  an  excess  of 
granulated  tin  in  the  solution. 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


INITIAL  FINlfoF  25  CENTS 


K 


3S- 


